| Classifying Gemstones
There are different ways of organizing the world of gemstones. The science of mineralogy sorts the various minerals into 10 different mineral classes, based on their crystal chemical composition. The classes include basic elements, sulfides, halides, oxides and hydroxides, carbonates and nitrates, phosphates, silicates and organic substances.
Within these classes are species. Each species has a well-defined chemical composition and a physical (usually crystalline) structure. Gemstone species includes corundum, quartz, beryl, zircon, spodumene, opal and zoisite. In some cases, minerals with a common structure but slightly different chemical formulae make up a group, such as the garnet group, the feldspar group or the tourmaline group.
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Sapphire (corundum) |
Peridot |
Ruby (corundum) |
Citrine (quartz) |
Each species may have one or more varieties. Gemstone varieties are less rigorously defined, but are usually distinguished by color or some distinctive optical phenomenon. Varieties of the corundum species includes ruby, star ruby, sapphire and star sapphire. The different colors of sapphire -- yellow, pink, green, white -- are sometimes referred to as distinct varieties of corundum.
Quartz has a large number of varieties, ranging from the familiar amethyst and citrine to less well known varieties such as chrysoprase. Other species, such as zoisite, have only a few varieties. The zoisite varieties include tanzanite, thulite and anyolite. Yet other species have no distinct varieties at all. Peridot is an example, since peridot occurs only in green.
The varieties of some species have developed such a strong identity of their own that many consumers fail to recognize them as belonging to one species. This is true of members of the beryl species: emerald, aquamarine, morganite and golden beryl.
In a rather different way of organizing gemstones, Walter Schumann provides a commercial classification that arranges gemstones into
"best-known gemstones" (such as ruby, sapphire and emerald);
"lesser-known gemstones" (such as andalusite, iolite and sphene);
"gemstones for collectors" (such as simpsonite, tremolite and pyrolusite);
"rocks as gemstones" (such as obsidian, alabaster and agalmatolite) ); and
"organic gemstones" (such as coral, amber and pearl). Schumann organizes his well-known book, Gemstones of the World around this scheme.
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