Gemology Hub | Gemstone & Mineral Identification Guides

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A coloured gem may have a pale, see-through centre or areas that look almost black. Both affect its appearance, but they do not have the same cause. What is a window? A window is an area where light passes through the gem instead of being returned towards the viewer. You may see the background through it, especially near the centre. Cutting angles and the material's refractive index—how it bends light—help determine whether this happens. There is no single ideal depth for every gemstone. Why dark patches need a closer look Darkness can have several causes. The material may absorb a lot of light. Some facets may reflect dark surroundings or the observer. Extinction is another cause, involving light that is not returned to the viewer after internal reflection. For that reason, calling every black patch “extinction” can be misleading. A simple viewing routine Look at the gem fac...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A beach can look pale one day and show dark streaks after the waves have moved the sand. These bands may contain a higher proportion of heavy mineral grains. Sand is a mixture Sand describes grain size, not one particular substance. A sample can contain several minerals that came from rocks worn down elsewhere. Rivers, wind, waves, tides, and coastal currents move and rework those grains. The grains do not all travel in the same way. Nature does the sorting Density, size, and shape affect how sediment is transported and deposited. Repeated movement can concentrate denser minerals into patches or thin bands. US Geological Survey studies of the Atlantic Coastal Plain describe heavy mineral sands containing minerals such as ilmenite, rutile, and zircon. Some deposits also contain minerals that carry rare earth elements. Does dark sand mean valuable gems? No. Colour alone cannot iden...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A pink or pale feldspar crystal may contain thin, lighter streaks. Some of these patterns are perthite: an intergrowth of two feldspars within what appears to be one grain. A mixture that separates while cooling At high temperatures, alkali feldspar can contain both sodium and potassium in its structure. As conditions change during cooling, an intermediate composition can separate into sodium-rich and potassium-rich parts. Geologists call this process exsolution. The separation happens within the solid material. It does not require the crystal to melt again. What the streaks contain In perthite, the main host is potassium-rich feldspar. The lighter intergrown material is commonly albite, a sodium-rich feldspar. The intergrowth may appear as thin lines, patches, or irregular bands. Some examples are visible by eye. Finer examples need a microscope, and very fine intergrowths requi...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. Some crystals look as though their centres have been scooped out. Instead of a flat face, they show a rim and a series of steps leading inward. This can be a growth form called a hopper crystal. The edges grow ahead In hopper growth, the edges advance faster than the centres of the faces. The result can be a recessed or stepped face rather than a completely filled surface. Mindat defines a hopper crystal by this difference between edge growth and face-centre growth. The term describes the shape; it is not the name of a separate mineral. Salt offers a clear example Researchers studying sodium chloride crystals have produced hopper forms from strongly supersaturated solutions. In simple terms, the solution contains a large amount of dissolved salt compared with what it can comfortably keep dissolved under those conditions. The experiments show that growth rate and the supply of mat...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A crystal's shape is a useful clue, but it does not always reveal the material now inside it. Sometimes one mineral changes or is replaced while its old outer form survives. The result is called a pseudomorph. An old shape with a new identity The word means false form. A pseudomorph can preserve the appearance of an earlier mineral even though its composition or internal structure has changed. Replacement can happen gradually. Other processes involve material coating an older crystal or filling a space left behind. The details matter when describing a particular specimen. A museum example The Australian Museum holds flower-like specimens of chrysocolla replacing azurite from the Dorothy Mine in Queensland. Their striking forms record a mineral change, rather than simply the normal growth shape of chrysocolla. What does “after” mean on a label? A label such as “chrysocolla aft...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A round stone can be tempting to open. Before expecting a sparkling centre, consider another possibility: it may be a concretion. What is a concretion? A concretion forms when dissolved minerals cement sediment together in a particular area. Cement can build around a small starting point, making a harder mass within softer surrounding rock. As the surrounding material wears away, the harder object may stand out or come loose. Some concretions look like balls, while others have less regular shapes. How is a geode different? A geode develops around a cavity, often with minerals lining the inside. The Dugway geodes in Utah, for example, formed in cavities in volcanic rock. Mineral-bearing water later deposited material inside them. A concretion is generally a cemented mass rather than a crystal-lined hollow. The two objects can look similar from outside, so roundness alone does not ...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. A wall of tall stone columns can look as though someone built it. At places such as Devils Postpile in California, the pattern formed naturally as lava cooled. Cooling creates cracks Basalt is a volcanic rock. As a lava flow loses heat and contracts, cracks can divide it into columns. Geologists call these fractures columnar joints. The cracks develop in relation to the cooling surfaces. This is why many exposed columns look upright, while other examples can lean or curve. Why so many have six sides Hexagonal columns are common, but the pattern is not perfectly uniform. Columns can have different numbers of sides. At Devils Postpile, the US Geological Survey reports that about 55 percent have six sides. That variation is useful to remember. Nature can produce an orderly pattern without making every part identical. Columns are not single crystals A large basalt column is a piece o...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. Rock layers do not always lie neatly on top of one another. In some sandstone, groups of thin lines slope across the larger layers. This pattern is called cross-bedding. How the sloping layers form Wind or flowing water moves sand into ripples and dunes. As a dune moves, grains collect on its sloping surface. New layers build over older ones, leaving a record of that movement. If the sand is later buried and becomes rock, parts of those sloping layers can survive. A cliff or road cutting may expose them in cross-section. An ancient desert example The Navajo Sandstone at Zion National Park preserves large groups of cross-beds formed in ancient windblown dunes. Their orientation helps geologists work out the directions in which the dunes moved. Does every cross-bed mean desert? No. Water can also create cross-bedding. A geologist considers the grain size, the shape and scale of the...

AI-generated educational illustration. Conceptual only; not a photograph of an actual specimen or location. Have you seen a pale cliff covered with dark streaks? The rock may not be dark all the way through. Its surface may carry a thin coating known as desert varnish. A coating on the rock Desert varnish is a broad name for coatings seen on many exposed rocks in dry regions. What looks like one simple layer may actually contain several kinds of surface deposits. Water moving over rock, windblown dust, and organisms living on the surface can all contribute material. The coating has a different composition from the rock beneath it. Why the colours vary Iron-rich components can produce red, orange, or brown colours. Manganese-rich rock varnish is often darker. Different mixtures help explain why one cliff shows several shades. Streaks may follow routes where water runs down a wall. This does not mean the whole cliff is made of the minerals found in its coating. Look at the surface and th...

AI-generated educational illustration of a fulgurite; not a photograph of a real specimen. A thin, rough tube in the sand may look like an old root. Sometimes it records a very different event: a lightning strike. Glass formed this way is called a fulgurite. How lightning makes glass When lightning travels through sand, it releases intense heat along a narrow path. Some material can vaporise, while nearby sand melts. As the melted material cools, it forms glass around the path of the strike. At Great Sand Dunes National Park in the United States, this process produces dark glass tubes. Their shape follows the lightning's route through the ground. Why the outside looks rough The outside can carry grains of sand, so the object may look more like a sandy twig than a shiny piece of glass. A broken end may reveal the hollow interior. These delicate tubes can break easily. What a find can tell you A confirmed fulgurite is evidence of a lightning strike. A tube shape alone, however, is no...

  Interested in gemstones and the science behind them? Gemology MCQs: Gemstones Study Guide offers a structured way to learn and review key gemology concepts through multiple-choice practice. The first edition includes 529 questions across 108 topics in a 177-page PDF , designed for beginner to intermediate learners. Questions cover gemstone formation, physical and optical properties, crystal shapes, inclusions, faceting, synthetic gems, imitations, enhancement, and identification. You’ll also find quizzes on many individual gemstones and varieties, including diamond, garnet, beryl, quartz, chalcedony, corundum, and tourmaline. A dedicated 50-question section on physical properties reviews topics such as hardness, cleavage, fracture, specific gravity, and carat weight. Answers and explanations support self-paced review. When a source question does not include a separate explanation, the guide identifies that clearly. Get the study guide The ebook is available as a digital PDF. V...

A gemstone can look different under two lamps without necessarily qualifying as alexandrite. Lighting changes the colours available to illuminate a stone. A narrow-spectrum lamp can therefore create a striking visual shift that needs careful interpretation. What makes alexandrite different? Alexandrite is a variety of chrysoberyl associated with a definite colour change caused by chromium. SSEF’s 2012 news slides compare this with a chrysoberyl that looked different under a particular LED but showed almost no main colour change between daylight and incandescent light. Why the comparison conditions matter A specialised lamp can emphasise only part of a gemstone’s response. The appearance under that lamp is interesting, but it should not replace a comparison under standard lighting conditions. The same caution applies to photographs whose lighting has not been explained. SSEF’s later public article reinforces the need to consider an appropriate colour change together with chromium-rela...

Radiocarbon research adds a time-related question to pearl testing: when did the sampled pearl material form? This can help investigate historical claims, but age and natural-versus-cultured formation are different questions. A recent pearl can be either natural or cultured. How the historical research worked SSEF’s 2013 article describes collaboration with ETH Zurich. A small amount of pearl material was removed and measured using accelerator mass spectrometry. The method used carbon-isotope information, including the increase linked to twentieth-century atmospheric nuclear testing. Unlike an X-ray image, this procedure consumes a sample. The article describes an early research project, so its sampling amount and service availability should not be assumed to apply to every current test. Why the environment matters Marine reservoir effects can influence the apparent age. Freshwater pearls may be especially difficult because older carbon from limestone can enter the growing pearl...

Some Kashmir sapphires are famous for a rich blue colour with a soft, velvety appearance. However, other deposits can produce sapphires that look similar. A beautiful visual effect is not enough to establish Kashmir origin. What the laboratory compares SSEF’s 2013 article discusses microscopic growth features, inclusions, chemistry and spectroscopy. Fine particles in some growth zones scatter light and contribute to the characteristic appearance. The shape and arrangement of internal features can provide useful evidence. The article specifically compares Kashmir and Madagascar material. It describes similarities as well as subtle differences and explains why the evidence must be considered together. A single inclusion or one chemical number should not be turned into a universal origin rule. Origin does not guarantee quality Every deposit produces a range of material. SSEF stresses that a Kashmir label does not automatically mean an exceptional stone. Colour, transparency, cut an...

Some blue cabochons in antique jewellery may look like turquoise but consist of a different material. Odontolite is one historical example. It is associated with fossil dentine and was also marketed under names such as bone turquoise, which can easily cause confusion. The historic brooch investigation SSEF’s 2012 article describes six brooches containing mostly odontolite, along with some actual turquoise and glass. Microscopic examination and Raman analysis helped separate the materials. The finding shows why matching colour across a jewel does not prove matching identity. How odontolite becomes blue Odontolite contains mainly fluorapatite. The article explains that heating can change the state of manganese traces and create a blue appearance. This is different from simply assuming that every turquoise-coloured material is the mineral turquoise. Surface pores, banding and, in some specimens, features associated with ivory structure contributed to the investigation. Such observa...

Sapphire is often associated with blue, but the corundum family offers a much broader colour range. Yellow, pink, violet and green sapphires are examples of the fancy colours discussed in SSEF’s 2012 Facette. A colourless ideal composition can become colourful through very small changes. What produces the colours? Pure corundum is aluminium oxide. Small amounts of other elements can enter its crystal structure and change which wavelengths it absorbs. Chromium contributes to red ruby colour, while iron and titanium are important in many blue sapphires. The colour depends on more than the total amount of one element. Its chemical state, combinations with other elements and colour centres in the crystal can all matter. Some corundum also shows different colours or shades in different viewing directions, a property called pleochroism. Why colour names have boundaries SSEF discusses the difficulty of separating nearby colour categories, such as pink sapphire and ruby. Variety names i...

EDXRF stands for energy-dispersive X-ray fluorescence. It is a laboratory method for examining chemical elements in a gem without drilling a sampling hole. The long name describes a useful principle: elements emit characteristic X-rays when excited. How the measurement works An instrument exposes the sample to X-rays and measures the energies of the X-rays emitted in response. Peaks in the resulting spectrum help identify elements. With suitable calibration, the measurements can also provide information about their amounts. SSEF’s 2011 article explains why this approach is useful for both loose and mounted gems. The article describes the introduction of a particular instrument at that time; it is not a statement about the laboratory’s entire current equipment. What chemistry can help explain Chemical data can support gem identification, colour research and some geographic-origin investigations. It becomes more informative when combined with knowledge of the material and reliable...

Spinel is more than a red gemstone. Natural examples can be pink, violet, blue and many other shades. Its colour range reflects small differences in chemistry, while its crystal shape and geological setting add further interest for collectors. The role of colour-producing elements SSEF’s 2011 overview describes chemically pure spinel as colourless. Chromium can contribute pink and red, while iron, vanadium and occasionally cobalt influence other colours. The mix and amounts matter, so there is no single colour for all spinel. The article discusses classic and newer sources, including Myanmar, Sri Lanka, Tajikistan, Vietnam, Madagascar and Tanzania. These are historical source examples, not a complete list of every occurrence. Crystal shape is another clue Spinel has a cubic crystal structure and commonly forms octahedra, which resemble two pyramids joined at their bases. Twinning can create interesting shapes. A rough crystal can therefore be attractive even before it is faceted...

A pink sapphire with a violet component may become a cleaner pink after heating. Detecting this change can be difficult when the heating leaves little obvious damage inside the stone. The word low-temperature is relative: the temperatures involved are still extremely high. What the experiments showed SSEF’s 2010 article describes research on pink sapphires from Madagascar and East Africa. Heating around 800–1000°C could reduce the blue component. The researchers also studied samples at several lower and higher experimental temperatures. The important lesson is that familiar microscopic signs of intense heating can be scarce after lower-temperature treatment. A stone that lacks melted-looking inclusions is not automatically proven unheated. Zircon inclusions as evidence The study investigated zircon inclusions with Raman spectroscopy. Heating could alter their crystal condition and the shape of their spectral peaks. Measuring these changes provided evidence that was less obvious ...

A ruby’s inclusions can offer clues about its geological history. However, similar-looking inclusions can occur in stones from different places. Early research on rubies from Montepuez, Mozambique, makes this limitation especially easy to understand. What SSEF observed In its 2010 Facette, SSEF described a preliminary investigation of six small samples. Observations included twinning, fissures, crystal inclusions and fine rutile features. The article noted that the internal appearance could resemble Mogok-type rubies from Myanmar. This similarity is valuable for learning, but it does not make the two origins interchangeable. An inclusion pattern associated with a famous deposit may also occur elsewhere. The 2012 SSEF news slides reinforce this point with further Mozambique ruby examples. Why one inclusion cannot locate a mine A laboratory compares multiple microscopic features with chemical and spectroscopic evidence and documented reference samples. The strength of an origin op...