Gemstone : Gem 

Gemstone is a mineral, rock (as in lapis lazuli) or petrified material that when cut 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, crystalline rhodochrosite[?], but are exhibited in museums and sought by collectors.

Some gemstones are manufactured to imitate other gemstones, for example cubic zirconia is a synthetic diamond substitute. The imitations copy the look and colour of the real stone but possess neither their chemical nor physical characteristics. However, synthetic gemstones are not necessarily imitation. For example, diamond, ruby, sapphire and emerald can be manufactured in labs, which possess identical chemical and physical characteristics as the genuine article. Artificial corundums including ruby and sapphire are very common and they cost only a fraction of the natural stones. Artificial diamonds are manufactured as industrial abrasives. However, artificial diamonds in gemstone-quality on the other hand are still too costly to manufacture.

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). Larger version

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 shape 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.

In the jewellery industry, gemstones are classified into precious stones and semi-precious stones. Only four types of gemstones are considered precious. They are:

• Diamond
• Ruby
• Sapphire
• Emerald

There is a large number of semi-precious stones. These include:

• Agate
• Alexandrite
• Amethyst
• Aquamarine
• Beryl
• Citrine
• Garnet
• Olivine
• Opal
• Spinel
• Tanzanite
• Topaz
• Turquoise
• Tourmaline
• Zircon

Artificial stones used as gem stones include:

• Cubic zirconia
• Synthetic Corundum

Not technically stones, but used in the same capacity:

• Amber
• Coral
• Jet
• Pearl

Gemstones are described and differentiated by gemologists by certain technical specifications. First, what is it made of, its composition. Diamonds for example are made of carbon, rubies of Aluminum 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 come in different "varieties", for example corundum is found as ruby, and sapphires of various colors; diamond in various Types; beryl as emerald, aquamarine, bixbite (red), goshenite (colorless), heliodor (yellow), morganite (pink).

Gems have a certain refractive index, a certain dispersion, a certain specific gravity, a certain hardness, a certain cleavage, a certain fracture, a certain lustre. They may exhibit pleochroism of a certain sort, or double refraction to a certain degree and have an optic sign[?]. They may have a certain luminescence and a distinctive absorption spectrum[?].

Certain material may be present as inclusions. And the gem may occur in certain locations, "occurrence." Gems from different locations may display different characteristics which may aid in identification.

External Link

• A list of misleading gemstone names (http://www.gemscape.com/html/misnomer.htm)
• manufactured gemstones (http://www.chathamcrystals.com/)

See also: List of minerals

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