GIA's Gübelin Gem Project
GIA studied more than 400 important gemstones from the Gübelin Collection and is committed to sharing this repository of gemmological information. This work is a valuable resource for all students, gemmologists and researchers.
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Chemically pure beryl is colourless, but trace elements give rise to green, blue and pink/red colours. The most important variety is green emerald (coloured by chromium or vanadium), followed by blue aquamarine and yellow heliodor (both coloured by iron) and pink morganite (coloured by manganese). Some beryls fashioned as cabochons exhibit asterism (a star-shaped pattern of reflections) or chatoyancy (a “cat’s-eye” seen in reflected light).
Ruby and sapphire (usually blue, but also in every other colour) have been the most important coloured gemstones for several thousand years. Originating historically in south-east and central Asia, and more recently in eastern Africa, these coloured varieties of the mineral corundum have been much sought as gems because of their rarity, colour and durability. Gem corundum can display asterism and chatoyancy due to the presence of oriented mineral inclusions, and in some cases, a change of colour when viewed under different light sources.
Often thought of as a deep red gemstone, garnet can also be yellow, orange, green or brown – any colour except blue. There are six common garnet minerals – almandine, andradite, grossular, pyrope, spessartine and uvarovite – and several rare species. Gem varieties include green demantoid and tsavorite, orangy red hessonite and pink/red rhodolite. Besides brilliance and attractive colours, garnets sometimes exhibit optical phenomena such as asterism (a star-shaped pattern of reflections), chatoyancy (a “cat’s-eye” seen in reflected light), or a colour change when viewed under different types of lighting.
Found in nearly every colour – most notably red, pink and blue – spinels are popular gemstones because of their abundance, moderate cost and attractiveness. Red spinel has often been mistaken for ruby. Some spinel exhibits asterism (a star-shaped pattern of reflections), chatoyancy (a “cat’s-eye” effect seen in reflected light) or a colour change when viewed alternately under incandescent and fluorescent lighting.
Tourmalines have a very complex chemical composition, and more than a dozen mineral species are recognised within this group. Their highly variable chemical composition gives rise to a wide range of colours. Among the many varieties of elbaite tourmaline are verdelite (green), indicolite (blue) and rubellite (red). Some tourmaline specimens display multiple colours. Optical effects include pleochroism (a change of colour when the gemstone is rotated) and chatoyancy (a “cat’s eye” seen in reflected light).
Although colourless when chemically pure, zircon is typically yellow-brown to brown. It also comes in red, blue, purple, and green. Because of its high brilliance and dispersion, colourless zircon has sometimes been used as a substitute for diamond in jewellery.
Among the collection assembled by Dr Edward Gübelin are a number of uncommon gemstones from many worldwide localities. Documents for these gems present information on gemstones that range from occasional use in jewellery to those that are quite rare and seldom faceted because of the lack of suitable starting material.
