Science Fair Project Encyclopedia
A gemstone is a mineral, rock (as in lapis lazuli) or petrified material that when cut or faceted and polished is collectible or can be used in jewellery. Others are organic, such as amber (fossilised tree resin) and jet (a form of coal). Some beautiful gemstones are too soft or too fragile to be used in jewelry, for example, single-crystal rhodochrosite, but are exhibited in museums and are sought by collectors of mineral or crystal specimens.
Charateristics and classification
Gemstones are described and differentiated by gemmologists by certain technical specifications. First, what is it made of, its chemical composition. Diamonds for example are made of carbon (C), rubies of aluminium oxide (Al2O3). Next, many gems are crystals which are classified by crystal system such as cubic or trigonal or monoclinic. Another term used is habit, the form the gem is usually found in, for example diamonds which have a cubic crystal system are often found as octahedrons.
Gems are classified into different groups, species and varieties. For example, ruby is the red variety of the species corundum that belongs to the spinel or hematite group. Emerald (green), aquamarine (blue), bixbite (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.
Gems have a certain refractive index, a certain dispersion, a certain specific gravity, a certain hardness, a certain cleavage, a certain fracture, a certain luster. They may exhibit pleochroism of a sort, or double refraction to a degree and have an optic sign . They may have a certain luminescence and a distinctive absorption spectrum.
Certain material or flaws within a stone may be present as characteristic inclusions. And the gem may occur in certain locations, "occurrence." Gems from different locations may display different characteristics which may aid in identification.
A gemstone is prized especially for great beauty or perfection. Hence, appearance is almost the most important attribute of gemstones. Their beauty must also be able to stand the test of time; if a gemstone is scratched or crumbled, it loses its value instantly. Characteristics that make a stone beautiful or desirable are colour, unusual optical phenomena within the stone, an interesting inclusion such as a fossil, rarity, and sometimes the form of the natural crystal. It is unsurprising that diamond is prized highly as a gemstone, since it is the hardest substance known and is able to reflect light with fire and sparkle when faceted.
Traditionally, gemstones were classified into precious stones (cardinal gems) and semi-precious stones. The former category was largely determined by a history of ecclesiastical, devotional or ceremonial use and rarity imposed by the limits of known deposits and available collection methods. Only five types of gemstones were considered precious: diamond, ruby, sapphire, emerald, and amethyst. In current usage by gemmologists, all gems are considered precious, although four of the five original "cardinal gems" are usually—but not always—the most expensive.
Synthetic and artificial gemstones
Some gemstones are manufactured to imitate other gemstones. For example, cubic zirconia is a synthetic artificial diamond substitute composed of zirconium oxide. The imitations copy the look and colour of the real stone but possess neither their chemical nor physical characteristics. However, true synthetic gemstones are not necessarily imitation. For example, diamond, ruby, sapphire and emerald have been manufactured in labs, which possess very nearly identical chemical and physical characteristics as the genuine article. Synthetic corundums, including ruby and sapphire, are very common and they cost only a fraction of the natural stones. Smaller synthetic diamonds have been manufactured in large quantities as industrial abrasives for many years. Only recently, larger synthetic diamonds of gemstone quality, especially of the coloured variety, have been manufactured.
There are over 130 species of minerals that have been cut into gems with 50 species in common use. These include:
- Alexandrite and other varieties of chrysoberyl
- Amethyst (originally a "cardinal gem", but now no longer so, since huge quantities were discovered in Brazil and the price plummeted)
- Aquamarine and other varieties of beryl
- Feldspar (moonstone)
- Jade - jadeite and nephrite
- Lapis lazuli
- Olivine (Peridot)
- Opal (Girasol)
- Quartz and its varieties, such as tiger's-eye, citrine, agate, and amethyst
- Tanzanite and other varieties of zoisite
Artificial or synthetic materials used as gems include:
There are a number of organic materials used as gems, including:
- Jet (lignite)
- Mother of pearl
- Ammolite - from fossils formed from the shells of extinct ammonites.
- Weinstein, Michael, 1958, The World of Jewel Stones, Sheridan House, New York
- The Audubon Society Field Guide to North American Rocks and Minerals, 1978, New York, Alfred A. Knopf ISBN 0394502698
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York ISBN 0471805807
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