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Gemstone

From Academic Kids

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.

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Gem.pebbles.800pix.labelled.jpg
A selection of gemstone pebbles: made by tumbling rough rock with abrasive grit, in a rotating drum. The biggest pebble here is 40 mm long (1.6 inches).
Contents

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.

Value

A gemstone is prized especially for great beauty or perfection. Hence, appearance is almost the most important attribute of gemstones. 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. In terms of beauty, 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. However, it is important to understand that diamonds are far from rare with millions of carats mined each year.

Traditionally, common 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 valuable.

Another category of gemstones which is still in use is that of rare or unusual gemstones, generally meant to include those gemstones which occur so infrequently in gem quality that they are scarcely known except to connoisseurs. Here are included andalusite, axinite, cassiterite, clinohumite, iolite, among others all of which are durable, rare, and in better examples quite attractive.

Factors Influencing Esteem

The factors influencing the esteem in which gems are held are few in number but extremely important because they so directly affect value. These are attractiveness, durability, rarity, fashion, and size. They are not fixed in scope by any means and the predominance of one factor may compensate for shortcomings in another.

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.

Gemstone list

There are over 130 species of minerals that have been cut into gems with 50 species in common use. These include:

Minerals that infrequently occur in gem quality form:

  • Andalusite
  • Axinite
  • Cassiterite
  • Clinohumite
  • Iloite
  • Kornerupine

Artificial or synthetic materials used as gems include:

There are a number of organic materials used as gems, including:

Related topics

References

  • 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

External links

da:delsten de:Schmuckstein fr:Gemme (minralogie) he:אבן חן nl:Edelsteen pt:Gema (mineralogia) fi:Jalokivi zh:寶石

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