Corundum from Different Areas
One of the most famous localities for fine rubies and sapphires is the district around Mogok in upper Myanmar. The Mogok Stone Tract is an area of some 1000 km2. The dates of discovery are not known and the legends later associated with ruby finds can be discounted. The area produces fine ruby and blue sapphire today.
The quality of corundum varies among different areas, and some of the famous ones are listed below:
- Mogok, Myanmar: Known for producing high-quality rubies and blue sapphires.
- Montepuez, Mozambique: Known for producing rubies with vivid red color.
- Ilakaka, Madagascar: Known for producing sapphires with a range of colors, including blue, pink, and yellow.
- Ratnapura, Sri Lanka: Known for producing sapphires with a range of colors, including blue, yellow, and pink.
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| Trigon marking on pinacoid of sapphire |
The Burma Ruby Mines Company
After the annexation of upper Burma by the British in 1886, Edwin Streeter, the London Bond St jeweller, obtained a concession to work the ruby mines though his position was disputed. On regulariza- tion of the position and flotation of the Burma Ruby Mines Company (with over-subscription) mining was able to begin.
As so often seems the case when a new gem mining venture actually begins work and miners look at the terrain without the assistance of rose-coloured spectacles, extraction of the rubies was far from easy. The alluvial deposits in which the ruby crystals were contained lay under the Mogok settlement or beneath waterlogged valleys. Nonetheless in time mining was able to commence. No underground mining was involved. Up to 1908 the Company was able to pay a dividend but depression in the world market for gemstones and the exhaustion of the easily accessible deposits led to the closure of the mines in 1931.
Corundum from Different Areas
Corundum is a crystalline form of aluminum oxide with traces of iron, titanium, and chromium. It is one of the naturally transparent materials, but can have different colors when impurities are present. The transparent red corundum is a ruby, while other colors are called sapphires.
Corundum can be found in many areas around the world, including the United States, India, Myanmar, Sri Lanka, and Australia. The quality of corundum varies depending on the location and the mining conditions. In the United States, corundum was mined in North Carolina, Alabama, and Georgia. However, the availability of cheap Verneuil synthetic rubies in the US caused a price fall, affecting even the finest specimens. The Bombay market (where most lower-grade rubies were sold) suffered at least an equally drastic fall in activity. The First World War occupied attention from 1914 to 1918 and in 1925 the Company went into voluntary liquidation, surrendering its lease to the government.
Corundum from Different Areas
Corundum is a crystalline form of aluminum oxide with traces of iron, titanium, and chromium. It is one of the naturally transparent materials, but can have different colors when impurities are present. The transparent red corundum is a ruby, while other colors are called sapphires.
Corundum can be found in many areas around the world, including the United States, India, Myanmar, Sri Lanka, and Australia. The quality of corundum varies depending on the location and the mining conditions. In the United States, corundum was mined in North Carolina, Alabama, and Georgia. However, the availability of cheap Verneuil synthetic rubies in the US caused a price fall, affecting even the finest specimens. The Bombay market (where most lower-grade rubies were sold) suffered at least an equally drastic fall in activity. The First World War occupied attention from 1914 to 1918 and in 1925 the Company went into voluntary liquidation, surrendering its lease to the government.
Gem Mining in Burma
Native Mining in Burma
After the failure of the Company, mining in Burma reverted to native methods. Mining was allowed on payment of a monthly fee, which was higher if water and explosives were used. To obtain a mining licence in Burma, it was then necessary for one’s name to be on an arbitrary list of registered miners, but those on the list were often willing to lend their names to the less fortunate for a consideration.
The lack of regulations and the widespread use of child labor in the mines made mining in Burma a hazardous and exploitative industry. The working conditions were dangerous, and miners often suffered from lung diseases due to exposure to dust and toxic fumes. Furthermore, the use of explosives also put miners at risk of accidents and injuries.
Despite the challenges, mining in Burma continued for decades, and the country remained a major producer of rubies and sapphires. However, the industry faced criticism for its poor labor practices and environmental impact, and efforts were made to regulate the sector and improve working conditions.
In the dry season, miners use the pit mining method to extract gem-bearing alluvial gravel. They sink a shaft called a twinlon to reach the gravel and then drive tunnels as needed. However, during the wet season, miners need to use the lebin method instead, which involves constructing a timbered shaft.
Hillside Mining
When pit mining becomes too dangerous due to wet conditions, miners resort to hmyawdwin, which is a method of cutting holes into hillsides to extract gems. The mining process may lead to the discovery of caverns, known as loodwin or loo, which are then washed out to retrieve the fine specimens. Dredging streams tends to produce less fine material.
Mode of Occurrence of Ruby and Sapphire in Myanmar
When ruby is found in situ the host rock will be silica-deficient and most commonly a marble. This is the common mode of occurrence of ruby from Myanmar and Afghanistan. At Mogok ruby occurs in a metamorphosed limestone, traces of which provide calcite and apatite inclusions in the host crystals.
Sapphire from Myanmar is not apparently found in the marble and may have originated in syenites or in pegmatites though specimens are recovered from alluvial deposits as mentioned earlier.
Ruby has been found at other localities in Myanmar including the Sagyin Hills near Mandalay and Namseka, near Mainglon. The notable purple-cored rubies from Mong Hsu, 250 km east of Mandalay, are heavily fractured and the purple colour needs to be removed by heat treatment. The deposit was discovered in 1992.
Current State of Gem Mines in Myanmar
Since the nationalization of the Mogok mines in 1968, the government has been running several gem mines in the area. However, little has changed since the days of the British Company and smuggling is still persistent. The mines are operated similar to how the Burmese kings used to operate them, almost like a private concession.
By 1991, the government started allowing limited access to some gem mines by foreigners. However, little progress has been made in the industry despite the growing demand for high-quality gems. Smuggling is still rampant, and what is not smuggled abroad is sold at the yearly emporium in Rangoon.
Myanmar still produces some of the finest sapphires, but it has been overshadowed by the recent ruby discoveries. These discoveries have led to the growth of the ruby industry in the country.
The Mogok Area in Vietnam
The Mogok area in Vietnam has a long history of producing high-quality rubies and sapphires. In 1992, Kane and Kammerling wrote a paper in the Fall 1992 issue of Gems & Gemology detailing the then current status of the Mogok area. Their paper included very useful maps which are still relevant today.
Despite years of conflict and political instability, the gemstone industry in Vietnam has continued to thrive. The country is known for producing rubies, sapphires, spinels, and other gemstones of exceptional quality.
Today, Vietnam is a major player in the global gemstone market and is home to many skilled lapidaries and gemstone cutters. The Mogok area in Vietnam remains an important source of high-quality gemstones and continues to attract gemstone enthusiasts and collectors from around the world.
Ruby and Sapphire Deposits in Thailand and Cambodia
Thailand and Cambodia are important sources for rubies and sapphires. The ruby deposits are found along the Thai/Cambodia border in Chanthaburi and Trat province in Thailand, and Battambang province in Cambodia. The rubies tend to be brownish red and somewhat dark in colour, while the sapphires are generally dark blue. Fine yellow sapphires occur at Khao Ploi Waen and Bang Ka Cha, while blue sapphires occur mainly at Khao Ploi Waen, Bang Ka Cha and Bo Ram in Thailand and Pailin in Cambodia.
Rubies can be found in a wide area including Bo Rai, Nong Bon, Bo Waen and Tok Prom in Thailand and Pailin in Cambodia. The centre for the gem trade in this area is the Thai town of Chanthaburi. All gem deposits in this area are derived from iron-rich basalt. The high iron content of the mother rock carries over into the corundum, which is generally darker than the ideal. The stones are found in coarse yellow or brown sand, overlaying a bed of clay or basaltic rock. The beds are mostly within 2.5 m of the surface, but some of the mines are over 6 m deep. Mining is done using simple methods.
Blue sapphires are also found in western Thailand at the town of Bo Ploi, in Kanchanaburi province. In the late 1980s, mining at Bo Ploi was expanded dramatically.
Vietnam Rubies and Sapphires
Since 1983, Vietnam has become a major ruby producer. Vietnam rubies are strikingly similar to those from Myanmar due to the similar geology of the two areas. Some experts have even found Vietnam rubies to be at least equal in clarity of Myanmar stones. In a major survey published in the July 2004 issue of the Journal of Gemmology, Van Long et al. describe the geology of the ruby deposits in northern Vietnam. From 1991, blue, green, and yellow (BGY) sapphires have been recovered from alluvial deposits associated with basaltic-type rocks in southern Vietnam, and other types of corundum deposits have been found elsewhere in the country.
In northern Vietnam, the main ruby-producing sites are in Yen Bai province (the Luc Yen and Yen Bai mining districts) and the Quy Chau deposit in Nghe An province. The southern deposits of BGY sapphires are located at Dark Nong and Binh Thuan in the Dek Lak and Lam Dong provinces, respectively. Van Long et al. note a number of other sites.
It is pleasing to know that two exceptional ruby crystals weighing 2.58 and 1.96 kg have been designated as State Treasures and are inalienable. They were found in the placers of the Tan Huong and Truc Lac areas. Van Long et al. describe the gemmological properties of corundum (ruby and blue sapphire) from the Quy Chau area. Rubies range from purplish-red to purplish-pink with varying saturations and sapphires are blue. As a contrast, the authors describe ruby from the Yen Bai and Luc Yen deposits as showing all tones from red to pink; sapphires range from colorless through pale yellow to grey and blue. Rubies from Luc Yen are usually less saturated than those from Quy Chau. RI and DR are normal for corundum. A notably high SG of 4.08 is recorded for some Blue Green and Yellow sapphires from southern Vietnam.
Straight and angular parallel growth features are characteristic of rubies and sapphires from Luc Yen and Quy Chau. Some rubies have been reported to show swirliness long noted for Myanmar ruby. Van Long et al. give a list of solid inclusions for both rubies and sapphires by location of deposit: the commonest mineral inclusions in Luc Yen and Quy Chau rubies are calcite, dolomite, rutile, diaspore, and phlogopite. Rutile is found as short needles and also as twinned platy crystals. It may also appear as transparent or opaque orange-brown crystals trapped along growth zones. Zircon has been found in both types of deposit. Fluid inclusions may also contain euhedral negative crystals and flat, broad tubes, some capped by a mineral inclusion. All red and pink samples from Quy Chau and Luc Yen showed red under both types of UV, and some blue sapphires from the Khoan Tthong area of Luc Yen also showed red under both types of UV. Quy Chau produces some beautiful orange sapphires, the color of papaya, as well as dark blue sapphires and yellow chrysoberyl. At Luc Yen, some off-red and blue spinels have been reported and cobalt-blue spinels have also been reported in Vietnam, along with pink and green tourmaline and colorless topaz.
The trapiche effect seen in some Colombian emerald and described in the chapter on that species has also been noted in some ruby from Vietnam.
Kashmir Blue Sapphires (Padar Sapphire)
Blue sapphires from Kashmir are highly prized and considered fine enough by auction houses to advertise their place of origin, similar to Myanmar blue sapphire and ruby, and Colombian emerald.
The location and mines of Kashmir sapphires are not well-known. The best account of the area and its mines is C.S. Middlemiss's Mineral Survey Reports, Precious and Semi-Precious Gemstones of Jammu and Kashmir, 1931. The report includes a photograph and six folding sketch maps and is #4464 in the Sinkankas bibliography.
A later, though much less detailed record, published in 1934 by the Government of India, is A Sketch of the Geography and Geology of the Himalaya Mountains and Tibet, part IV, The Geology of the Himalaya. Here is an extract:
"Some fifty years ago the beautiful azure blue sapphire of Sumjam in Padar, Zanskar, Kishtwar tahsil, was accidentally discovered at an altitude of 15 000 feet among rocks which have since been determined as feldspathic pegmatite veins in actinolite-tremolite lenticles in the marble bands of the area. The actinolite-tremolite schist appears to have been a modification of the marble. Large quantities of excellent stones were found and they yielded considerable revenue to the Kashmir government.... Subsequently the actual source of the rock appeared to be exhausted though the placer deposit continued to yield a diminishing output. Later on, new veins of corundum, sapphire and pink corundum (ruby) were discovered in the area and its neighbourhood... in colour, the sapphire is of a pale china-blue tint but more generally it is a rich sky blue which in the best stones becomes extremely vivid. Occasionally a more slaty blue tint appears. The colour is irregularly and unevenly distributed in the sapphire crystals, being found in stripes and patches of different dimensions among the milky grey and colourless corundum. A reddish tint is also found, though somewhat rare. Where found it varies from pale pink to rosy red and in a few cases to carmine with a slight blue tone in the red."
Gübelin and Koivula describe and illustrate some of the characteristic inclusions of Kashmir sapphire in Photoatlas of Inclusions in Gemstones, second edition, 1992. They describe a 'milky turbulence' which is thought to be caused by a profusion of cavities and exsolutions which diffuse the incident light. Mineral inclusions are rare but the authors illustrate a fine crystal of tourmaline magnified by 32. Since blue sapphires from other places usually show a rather more defined inclusion pattern anyone testing a sapphire with no obvious inclusions should not rule out a possible Kashmir specimen.
Rubies in India
India is one of the countries that produce rubies. The gemstone is found in several Indian states. Facet-grade ruby has been reported from the Kangayam area of Tamil Nadu and from Karnataka. Although, the quality of rubies found in India is not high and many stones are treated in one way or another, star rubies are fairly common.
Pakistan Rubies
In Gemstones of Pakistan (1990) Kazmi and O’Donoghue consider the attractive if frequently calcite-included rubies from the Hunza area in the north of Pakistan. The ruby deposits are associated with the Karakoram suture zone, the stones occurring in metamorphosed recrys- tallized dolomite marble in a narrow belt extending from the Hunza valley to Ishkoman, a distance of more than 100 km. Crystals are well formed and range from pink to a fine red. Included material is cut into cabochons but some of the finest reds are high facet-grade. The writer (MO’D) who visited, with Dr Kazmi, the gem-bearing areas of Pakistan for the United Nations Industrial Development Organization in late 1985 examined some beautiful specimens both in situ and after fashion- ing and found that despite the inclusions many of the pink stones were most attractive.
Other mineral inclusions were dolomite, the potassium mica phlogo- pite which occurs as reddish brown flakes and feather-like greenish crys- tals of the calcium mica, margarite. Pyrite, pyrrhotite and apatite were also found. However, no trace of rutile has yet been reported so that no Pakistan star rubies are likely to occur (so far). The absence of rutile is of course useful in the diagnostics of locality. No sapphires have yet been reported from Pakistan. Some Pakistan rubies show a distinct phospho- rescence following X-ray irradiation, a phenomenon most commonly seen in some synthetic rubies. Readers are advised to consult Bender and Raza, Geology of Pakistan (1995; ISBN 3443110258).
Afghanistan Ruby/Sapphire Deposits
Bowersox and Chamerblin (1995) in Gemstones of Afghanistan give some details of the Jegdalek (spellings vary as with many gem-producing areas worldwide) ruby deposits which are, again as always, not easy to link with the mines described by previous authorities.
Two papers by Griesbach in Records of the Geological Survey of India (Afghan and Persian field notes (Records, vol. 19, pp. 48–68, 1886) and Field notes to accompany a geological sketch map of Afghanistan and Khorassan (Records, vol. 20 no. 2, 93 103, 1887)) are quoted by Bowersox and Chamberlin as well as a book by Catelle (1903) – although omitted from the list of suggested reading this must be Precious Stones, A Book of Reference [Sinkankas #1190] which was well thought of in its day.
Bowersox and Chamberlin place the Jegdalek ruby deposit in the southern portion of the Sorobi district, 60 km south-east of Kabul in the Jegdalek river valley. They attribute ruby formation to the contact meta- somatism of ultra-acid granites found in contact with Precambrian car- bonate and magnesium rocks in the Pamir-Nuristan median mass.
Ruby mineralization is irregularly developed. Gem ruby is found in situ in interstratified Proterozoic marble and gneisses intruded by granitic rocks from the Oligocene. The ruby-bearing marble is notably coarse-grained. Ruby crystals are also found in a white dolomitic granular limestone. Mining, as attested by Gübelin in Gems & Gemology, 18(3) 123–29, 1982, is primitive.
Bowersox in Gems & Gemology, 21(4) 192–204, 1985, notes that the most noticeable inclusions seem to be small spots or zones of a strong blue colour; their shape is mostly trigonal or hexagonal in outline. Some stones show parallel blue bands. A sharp division between red body colour and blue patch is highly characteristic of Jegdalek rubies. Crystals, apparently of calcite, are common.
A personal communication from Themelis to Bowersox states that the blue inclusions are effectively removed by heating, leaving the stone pink.
Corundum with a wide range of colours is found in south-west Sri Lanka. A good account may be found in Gübelin, Die Edelsteine der Insel Ceylon (1968 – no English translation, privately published but with apparently a very limited one-time commercial distribution, now very rare: Sinkankas #2575). Sinkankas (1993) makes the point that almost all the literature pertaining to the gemstone mining in Sri Lanka is in the form of papers in journals, there being no monographic survey. This is still the case in late 2004.
Sri Lankan ruby inclines to pink rather than crimson but stones are very bright and lively – this applies also to blue sapphire. Star rubies and blue sapphires can be very beautiful. Fine examples include the Rosser Reeves star ruby of 138.7 ct (Smithsonian Institution) and the 392 ct star sapphire owned by the State Gem Corporation of Sri Lanka is exceptional. Mining is simple and any method of reaching the illam (gem gravels, perhaps 15 m down) is used, sites being chosen by chance finds of pebbles at the surface.
Virtually all Sri Lankan gems are cut and polished locally. Much blue sapphire shows the best or only blue in one part of the crystal; lapidaries manage to place the blue in the culet (bottom) area of the faceted stone so that the blue shows through the table though not in any other direction – such stones are intriguing and beautiful. Colour change (blue to purple) may be seen in some sapphires.
Sri Lankan ruby shows inclusions distinctive enough for the location to be determined. Flakes of ‘biotite’ (this name is now taken as a series name for dark lithium-free mica rather than as an individual species name) are very characteristic and rutile needles are generally longer and more slender than those seen in Myanmar stones. Grains of metamict zir- con are accompanied by dark tension haloes and liquid inclusions form feather-like patterns. Sri Lankan blue sapphire shows distinctive ‘arrow- heads’ of rutile which form V-shaped structures; the feldspar group min- eral albite, apatite and hematite may also be found. Gübelin and Koivula (1992) illustrate albite crystals forming ‘comet-tails’ (an assemblage more familiar in synthetic corundum).
Sri Lankan blue sapphires on the market today may very well have begun life as milky and colourless. The name geuda has been used for both the colourless material and the blue crystals resulting when it is heated. The treatment of gemstones for colour improvement is a major concern in the gem trade today, difficulties arising as much over disclosure as identification. Scientific and practical matters in this context are excel- lently covered in monographs by Nassau (Gemstone Enhancement, second edition, 1994; ISBN 0750617977) and Themelis (The Heat Treatment of Rubies and Sapphires, 1992). Further details can be found in Chapter 30. Today it is not easy to find an untreated sapphire in Sri Lankan markets.
Blue Sapphires from Wutu, Changle County, China
China has a rich deposit of blue sapphires found in alluvial deposits and in situ near Wutu, Changle county, Shandong Province, as reported in a paper in the Winter 1992 issue of Gems & Gemology. These sapphires have different hues of dark blue, blue, greenish blue and yellow, and are mostly hosted by basalt. The most common trace elements found in these sapphires are iron, gallium, titanium, cobalt, and vanadium. Gemmological properties of these sapphires are similar to other basalt-hosted sapphires. The mineral inclusions in these sapphires include uranium- and thorium-rich zircon, titanium-rich columbite, sodium feldspar, apatite, ilmenite, and magnesium-iron spinel. The orange-red zircon inclusions have reached 1 mm in size, while the black metallic lustre columbite inclusions are also present.
Australia Corundum
Corundum deposits in Australia were first discovered in the middle of the 19th century, but commercial production of sapphires did not begin until after World War I. Sapphires were found in the alluvium of streams on Frazer’s Creek near the town of Inverell in the New England area, and the name Sapphire was taken up by the settlement which grew up. As more stones were coming from deposits in Southeast Asia, production tailed off in the 1930s. However, the search for and marketing of New South Wales material took on new life as prices for Australian sapphires improved.
In general, Australian sapphires are either dark or heavily included, but often both. Nonetheless, many stones may be considered the best of their color, particularly green specimens. Green and yellow stones, in particular, show a clear absorption spectrum, though the iron content inhibits the yellow stones from fluorescing.
New England Sapphire Production
The New England district of New South Wales produced sapphires that are dark blue or dark green with intermediate shades of either color. Yellow crystals are rare. In alluvial deposits, crystals are found as fragments with the common form of tapering hexagonal prisms, though some well-rounded crystals also occur. Color zoning is pronounced, and hollow cavities, rutile needles, and cavities are common.
Anakie Sapphire Deposit
The Anakie sapphire deposit in central Queensland was proclaimed in 1902 as a recognized commercial field. While corundum fragments have been found in the basalt, crystals are recovered from alluvial sources. The proportion of gem to non-gem-quality material is low, but a wide range of colors has been found, including golden yellow and star stones. The very occasional ruby and color change crystal have been reported.
Harts Range Ruby Crystals
In 1978, flattish ruby crystals, rubies of poor cabochon grade, were recovered from a shallow subsurface deposit on the Harts Range, northeast of Alice Springs, Northern Territory.
USA Corundum
To European gemmologists (this one [MO’D] at any rate), Americans seem seriously to under-value their gem minerals! Some of the sapphires found in North America rank with the most beautiful yet recovered. Two different types of occurrence, in situ (hard rock) and alluvial deposits, both produce specimens of high individuality.
Sapphires of Yogo, Montana
In 1900 Weed and Pirson produced Geology of the Little Belt Mountains, Montana, the study forming part 3 of the 20th Annual Report of the United States Geological Survey, 1898–99. Discussing the transition from analcite- basalt to minette [orthoclase-biotite lamprophyre consisting of biotite and subordinate diopside phenocrysts in a groundmass of orthoclase or sanidine]. Weed and Pirsson study the sapphire-bearing rock of Yogo Gulch, Judith Basin county. The first report of the occurrence was published in American Journal of Science, 4th series, vol. IV, p. 421. Stepen M. Voynick’s Yogo, the Great American Sapphire (1990; ISBN 087842217X) gives an excellent account of the history of the area as do the three volumes of Gemstones of North America by John Sinkankas.
Highly characteristic sapphire crystals occur in a dark silica-poor lamprophyre dike consisting of biotite and pyroxene group minerals. The dike intrudes the Madison limestone and though only a few feet wide, extends for at least five miles.
The history of the deposit is well worth reading, especially as there are few important examples of gem corundum crystals being mined from their parent rock. At one time British and American companies worked different ends of the dike and a business venture, The New Mine Sapphire Syndicate, was set up and in 1914 produced a beautiful and now very rare advertising booklet, A Royal Gem, by Arthur Tremayne. The beauty of this booklet [Sinkankas #4741] is in its (blue) colour illustrations of Yogo sapphires in their cut and rough state and also as set in jewellery. For some time, sapphires from Yogo were handled by the London firm of Johnson, Walker and Tolhurst who retain illustrated cata- logues, featuring Yogo sapphires, at the present time.
Yogo Sapphires
Yogo sapphires are highly prized for their rich blue color, often compared to the color of a Montana sky. They are also known for their exceptional clarity and rarity. Yogo sapphires are only found in Yogo Gulch, Montana, making them a unique and sought-after gemstone.
The Yogo sapphire is a type of blue sapphire found in Yogo Gulch, Montana. The specimens are characterized by flattened crystals with a regular outline that is hard to detect as prism faces are virtually absent. Yogo sapphires are almost recognizable from their exceptionally bright but deep blue color. Lilac and purple stones can also be beautiful, and a very few rubies have been found.
Sapphires are generally free from particularly characteristic inclusions, but Yogo sapphires have an inclusion pattern reminiscent of Thai rubies; analcime crystallites with haloes of fluid drops. Some analcime crystals show cube-icositetrahedron combinations.
Yogo sapphires are the most commercially important of all the colored stones found in North America so far. Price guides for any gem species are notably unreliable owing to the market’s inevitable volatility but, according to Miller and Sinkankas in the Standard Catalog of Gem Values (second edition, 1984), fine blue Yogo sapphires weighing 0.5–2 ct can range from US$ 1000 to 3500.
In The 1922 Handbook and Descriptive Catalog of the Collections of Gems and Precious Stones in the United States National Museum, Merrill lists only two blue sapphires under the heading Yogo Gulch, and in Corundum Deposits of Montana (United States Geological Survey Bulletin 983, 1952), Clabaugh mentions a price of $75 a carat for the larger Yogo sapphires when faceted. This is a vital report for anyone interested in Montana corundum in general.
In the book, 'Gemstones, Quality and Value', Yasukazu Suwa explains that Montana blue sapphires are usually watery in appearance and hence, do not command high prices. However, this does not apply to Yogo sapphires which are far from watery in appearance. Crystals from Yogo are illustrated in extraLapis 15 (1998) and show the crystallographic nature of eight typical specimens.
Wide range of coloured sapphires can be found in the gravels of the Missouri River in the vicinity of Helena, Montana. Although the colours can be very attractive, most of the production is routinely heated to give brighter colours. Colours usually sold are blue, green and yellow. A fine ‘hot pink’ is achieved by heating and, in general, the treated stones are most attractive.
Details of the results obtained by heating can be found in papers by John L. Emmett and Troy R. Douthit, who have made a close study of heating methods and their results. Yogo sapphires are not heated.
The main working sites for Montana sapphires are Rock Creek, Dry Cottonwood Creek and Eldorado Bar. A number of individuals and companies have made efforts over the years to establish a serious and profitable trade in the heated sapphires, but there has been limited and local success. The sapphires are not well-known in Europe or even in the US, although the position could improve if only the supply of sapphires from South and Southeast Asia and Australia was not so well established.
Sapphires, apart from Yogo specimens, known to be from Montana are so routinely heated that disclosure is probably not needed, although if there were to be a regular export of sapphires from North America, some form of certification might be called for. When not heated, the sapphires from the gem gravels may show negative crystals occupied in some instances by natural glass and one or more gas bubbles. Heating alters inclusions and obscures their outlines.
In North Carolina, rubies of fair quality have been found in Macon County, at Cowee Creek. Ruby crystals are found in gravels with red garnet. Rubies with rutile inclusions have been cut to give star stones of good quality.
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Ruby Mining in Tsavo West National Park, Kenya
In 1973 American geologists Tim Miller and John Saul discovered ruby in Tsavo West National Park, in the Mangari area. As so often happens, facet- grade crystals were much less plentiful than heavily included cabochon- only material. Crystals in some cases were well formed and would have made desirable collectors’ specimens had many not evaded the cabochon cutters and perhaps exporters. The Kenya government of the time approved the mines, though ownership disputes took time to resolve. Private owners eventually regained control and good-quality cabochon-grade and a few facet-grade crystals are being recovered. Most crystals are sent to Thailand for cutting and treatment.
Tanzania Corundum
Corundum from Tanzania occurs in a wide range of colours including orange, purple, mauve and yellow as well as red, green and blue. In gen- eral, the colours can be said to be pastel rather than well defined but are particularly attractive for this reason. Ruby inclusions in a bright green chromiferous rock make good carving material and have been given the vernacular name anyolite.
Of greater importance are the varieties of sapphire from the Umba river valley in north-eastern Tanzania, near the Kenya border where sap- phires are mined from several pits in and around a greyish green serpen- tine pipe. The sapphires and rubies of Umba occur in various colours, often of a pastel shade which is very attractive. Purple and mauve and orange sapphires, in particular, are beautiful.
In the Fall 1991 issue of Gems & Gemology, Hänni and Schmetzer report the occurrence of ruby in parcels of red spinels said to be from the Morogoro area of Tanzania. The colour and inclusions were similar to some found in Myanmar ruby; inhomogenieties in growth zoning parallel to distinct faces and irregular swirls; sets of twin lamellae parallel to one, two or three rhombohedral faces; clouds of rutile crystals and coarse rutile; octahedral inclusions with slightly rounded solid material (spinel) or nega- tive crystals looking similar. Zircon, apatite and pyrope have also been identified. The potential for asterism has been found in some specimens.
The mines are situated in the western Uluguru Mountains. An excel- lent account of ruby mining in Tanzania appeared in extraLapis 15, Rubin, Saphir, Korund, 1998. See also zwaan garnet, corundum and other gem minerals from Umba in Scripta Geologica 20, pp. 1–4, 1974.
Nigeria Corundum
Starting in the late 1970s, corundums of a blue, green and yellow colour began appearing in world markets. These are basalt-derived stones and tend to be dark in colour. The mines are located in the Kaduna district.
Malawi Corundum Mining
Malawi corundum has been known since at least 1958 when the Nyasaland Geological Survey reported on it. Regular mining of the deposits at Chimwadzulu Hill is more recent. The area is 145 km south-east of the capital city, Lilongwe. The area comprises mainly metamorphosed ultramafic rocks with an exposed surface and an exposed surface diameter of about 1 km. The surrounding county rocks are Precambrian metasediments metamorphosed into schists and gneisses.
The bedrock is deeply weathered on the hill with a surface layer of iron-rich red porous clay. Corundum is found in the topsoil and there is good evidence of bedrock resources. Minerals in aggregate found with the corundum include quartz, magnetite, hematite, and chromite and separation work is necessary for extraction of corundum. David Hargreaves, Chairman and CEO of Minex (Pvt.) Ltd and the owner and operator of the mine, reports that more than 50% of the corundum is magnetic and this property may be found useful in separation.
Corundum-bearing soil is scraped off the bedrock and sent for processing. When Minex began developing the mine in 1995 about 10% of the ore body had been worked. Recovery potential seems promising. The area had been described as a sapphire deposit in which the occasional ruby was found. Naturally colored sapphire crystals of quality were rare and seemed excellent candidates for heat treatment. Conventional heat-treatment methods, however, did not achieve good color.
Fortunately, the discovery of ruby in natural colors ranging from pink to purple with some orange padparadschah helped the operation to survive. Further experimental work on heating the sapphires has now produced some good blue, yellow, and fancy colors though no red stones were achieved, nor could naturally red crystals be enhanced. Taking into account the high probability that the gem-buying public will turn to stones whose color can be shown to be natural, the potential of Chimwadzulu corundum is good.
Chimwadzulu corundum can be described as ranging from a true pink to a very dark purplish blue. Stones weighing more than 1 ct sell on their individual merits though melee, high in volume and with sales vital, needs color consistency. For this reason, stones have been grouped into nine categories; ruby, padparadschah, purple, pink, yellow, teal, blue, cognac, and fancy.
Malawi Corundum from Columbia Gem House
Locality information is always useful when selling gemstones. A paper by Professor Andy Rankin in The Journal of Gemmology in 2003 described the Chimwadzulu material and the results of heating trails. The ratios (wt% oxides) of Cr/Ga vs Fe/Cr for Chimwadzulu and Thai and Kampuchean material shows only a minor overlap: there is no overlap with Mogok, Mong Hsu, Vatomandry and Longido.
Our Malawi corundum is sourced from the Chimwadzulu mine and is of the highest quality. Contact us to learn more about our selection of gems.
African Sources of Sapphires
Sapphires of various colours have been reported from Namibia. Blue sapphire has been found in the Beraketa region of Madagascar where it occurs with feldspar; blue sapphire is also reported (extra Lapis English no. 1, 2001) from a skarn south-east of Andranondambo. Blue sapphire of moderate quality is found in some abundance in the Ilakaka river; the town of Ilakaka has become a major gemstone trading centre. Readers should consult Lapis for June 1999 for a paper by Karl Schmetzer on Madagascar corundum. Fancy sapphires (not ruby or blue sapphire) appear to be characteristic of African deposits. Crystals with a colour change (the colours varying) may become another African speciality.
Small deposits of corundum of various colours have been reported from the Somabulu forest in south-western Zimbabwe and a deposit of sapphire has been reported from a pegmatite and in alluvial gravels in north-eastern Zimbabwe.
Dark blue sapphires have been found near Jauru, Mato Grosso, Brazil, and both ruby and sapphire in the Rio Mayo, a tributary of the Patia, and in the sands of the Platayaco, in the Caqueta area of Colombia.
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