Zircon {Jargoon, Hyacinth, Jacinth}
Note: Jargoon or jargon is a name applied by gemologists to those zircons which are fine enough to be cut as gemstones, but are not of the red color which characterizes the hyacinth or jacinth.
While zircon in fashioned form can be found in a wide range of colours the crystals are usually reddish brown and heat treatment is needed to produce the colourless, golden yellow and bright blue colours most seen in jewellery. Other colours, often very pleasing, might be called fancy zircons as the range is considerable. We should understand from the first that some zircons are much older than others and, like older people, behave in different ways. Over time some zircons have suffered a break- down of their crystal lattice to the extent that in extreme cases they are to all intents and purposes amorphous. They show little or no birefringence and their physical properties are different from those of other zircons. They are ‘low’ zircons and come virtually exclusively from the gem grav- els of Sri Lanka. Zircons from elsewhere, with their crystal lattice intact, are ‘high’ zircons. The name metamict (first used by Brögger in 1893) is applied to low zircons, which approach a glassy amorphous state, are almost always green and can be radioactive though gem specimens do not normally show this property to any significant extent.
The progressive disintegration of the crystal
lattice in metamict
zircon
arises from the presence of decaying nuclei of uranium and thorium, the process emitting alpha particles. With
each emission the nucleus recoils, causing
the displacement of lattice atoms. This process is described in Deer, Howie and Zussman, Rock-Forming Minerals, second
edition, vol. 1A, Orthosilicates, 1982.
Zircon is zirconium silicate ZrSiO4 and
forms prismatic crystals of the tetragonal
system with square cross-sections and is sometimes attract- ively terminated by pyramids; large
reddish brown crystals from Nigeria can
be especially attractive. Geniculate twins are also found. Crystals show a markedly oily lustre. Most gem zircon occurs as pebbles in gem gravels. Zircon is often brittle and faceted stones have traditionally
been kept separate from one another
so that their facets do not become ‘paper worn’.
Thai dealers for years used ‘zircon twists’ in which several stones were kept in screws of black paper.
The hardness is 7.5 and the SG 4.5–4.7. The RI for the ordinary and extraordinary rays is 1.925–1.961 and 1.980–2.015 respectively with a birefringence of 0.059, uniaxial positive. Metamict zircon may have SG around 4 and RI as low as 1.78 with scarcely perceptible birefringence. The dispersion, seen at its best in the heat-treated colourless material when faceted, is about 0.039. Properties to some extent identify the degree of metamictization and gem zircons have been classed as high, low and intermediate, the absorption spectrum often providing a useful indication of a specimen’s position on this arbitrary scale. Zircon shows very weak pleochroism in general though blue heat-treated stones will give a blue and near-colourless response.
In high zircon and particularly those from Myanmar there is a beauti- ful and near-diagnostic uranium absorption spectrum with fine though strong lines and narrow bands extending across the entire width. The strongest band is at 635.5 nm. This phenomenon was first described by Church, with the spectrum of almandine, in The Student’s Guide and Intellectual Observer, 1866. Myanmar greenish brown specimens may contain at least 40 lines and bands. Details of the absorption spectra of gem zircons were collected by B.W. Anderson and C.J. Payne over many years between and after the Second World War while working at the London Gem Testing Laboratory. Their work was collected by Anderson in MS notes, copies of which are held by GemA in their library in London. Between 1954 and 1957 the journal The Gemmologist published some of these notes and in 1998 R.K. Mitchell edited them for a mono- graph The Spectroscope and Gemmology, GemStone Press, Woodstock, Vermont and Robert Hale, London; ISBN 071980261X. In the section on zircon Anderson & Payne list bands shown by different zircons, making the point that some are best seen in the ordinary or extraordinary rays. Some showed up best on a photograph taken with a small grating spec- trograph using plates sensitive to the deep red. Not only do zircons show a variety of absorption bands and lines corresponding to their low or high state; when heated, some specimens show an anomalous spec- trum. To an extent the absorption spectrum may give some clue to ori- gin as specimens from Uralla, New South Wales, show only a few bands, and red specimens from sites in the Auvergne, France, have shown no absorption in the visible. When zircons from the Indo-China region are heated to give commercial colourless, golden yellow and blue their absorption spectrum is very weak and may show only the persistent band at 653.5 nm.
The main bands in the spectrum of zircon are at 691, 683, 662.5, 660.5,653.5 (the strongest and most persistent), 621, 615, 589.5, 562.5, 537.5, 516, 484, 460 and 432.7 nm. Metamict zircons show a woolly band at 653.5 nm.
Heating these zircons may cause this band to sharpen and other lines to appear. There are some rare variations of the low-type absorption spectrum, one of which shows three broad strong bands in the red at 691, 669 and 653.5 nm, the centre band being the strongest. Another type, which curiously has been found only in zircons with an RI of 1.82 and an SG of 3.98, shows a vague band at 655 nm and another at 520 nm.
Doubling of back facets in Zircon
Fluorescence
Some zircons show cathodoluminescence and thermoluminescence but the responses are not usually valuable in gemmological testing. Intermediate types may give an orange incandescence in a Bunsen flame – thorium has been suggested as the cause. Neutron irradiation of yellow crystals causes their colour to change to bright green (the colour of metamict zircon). Subjecting zircon to any form of irradiation may cause them to discolour. This is especially true of the colourless and blue specimens whose colour has been arrived at by heating. Though gemmologists speak of colour fad- ing this is, like so many other pronouncements, incorrect; dark patches appear in the stone and spread as the treatment continues. Heating usually restores their original appearance though not, it can be imagined until some heart-searching has taken place.
Heating
Both the low and intermediate zircons when heated to about 1450 °C may achieve an SG of 4.7 with high-type RIs and sharper absorption lines. Heating the reddish brown Indo-Chinese crystals gives blue, colourless or yellow specimens according to whether or not an oxidiz- ing atmosphere is employed. Colourless and sky-blue zircons need reducing conditions while golden yellow colours are achieved in oxi- dizing conditions. Gem zircon is usually the high type.
The best account
of treatment can be found in Nassau,
Gemstone Enhancement, second edition,
Butterworth Heinemann, Oxford,
1994; ISBN 0750617977. Nassau
reminds readers that many references to the methods used in heating
zircon can be found in Eppler, Das Geheimnis
des Zircons, in Goldschmiede Zeitung 51,
531 (1936) and by Buckingham in the Journal of Gemmology 2,
177 (1950).
Zircon Treatment the traditional way 2017 - Princess Gemstones
Inclusions
Metamict zircon shows the most characteristic inclusions. Most prom- inent are tension fissures meeting at an angle of 57˚5 and probably due to the degeneration process. They echo the original crystal’s prism and dipyramidal faces. Also due to isotropization are disc-shaped tension fissures. Unequal isotropization may give rise to parallel stripes which also suggest tetragonal form (Figure 7.3). Ilmenite may be found in frac- tures and some healing fissures are reported.
Occurrence
Zircon is unusually common and widely distributed though most gem crystals occur in the gem gravels of Sri Lanka and some are found in the Mogok area of Myanmar and in Nigeria. The red zircon of Expailly, Auvergne, France, has already been mentioned. Gem crystals in a variety of colours and sizes have been found in the Mud Tank area of the Harts Range, Northern Territory, Australia. These crystals appear to have under- gone less structural damage than the Sri Lankan material. Fine crystals are reported from Slyudanka, Irkutsk Oblast, Russia, and from Seiland Is., Alta Fjord, northern Norway; this area has produced beautiful orange crystals up to 2.5 inches in length.
The gem zircon locations in South-east Asia include the Pailin district of Kampuchea and the Chiang Mai district, Thailand. The zircon- producing sites overlap political boundaries in many cases so that names can be hard to locate. In all cases material is sent to Bangkok for fashioning.
Reddish brown zircon is reported from decomposed alkali basalts near Jemaal Kaduna, Nigeria. This is a source of fine crystals.
Fashioning
In order to obtain the best optical effect with zircon, the stones are some- times cut into a modified brilliant cut which has a second set of pavilion facets. The octagonal and four-sided trap cuts are now extensively used for specimen zircons of the blue and golden colours, for some of the greens and yellows, and also the browns of natural colour. The natural coloured stones are, however, more usually cut in the mixed-cut style. The high birefringence can affect the appearance of the finished stone if care is not taken by the lapidary.
A basaltic rock with groups of flower-like radiating crystals of zircon and xenotime-(Y), (YPO4), is found in the hill Maru-Yama, just north of Mount Funabuse, Gifu prefecture, Japan. The rock is locally called kiku- ishi (chrysanthemum stone). A similar rock is found, usually as boulders, in Vancouver Island of British Columbia, Canada.
Small crystals of zircon have been grown hydrothermally and are described in the chapter on synthetics
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