A gemstone or
gem (also called a precious or semi-precious stone) is a piece of
attractive mineral, whichwhen cut and polishedis used to
make jewelry or other adornments. However certain rocks, (such as
lapis-lazuli) and organic materials (such as amber or jet) are not
minerals, but are still used for jewelry, and are therefore often
considered to be gemstones as well. Most gemstones are hard, but some
soft minerals are used in jewelry because of their lustre or other
physical properties that have aesthetic value. Rarity is another
characteristic that lends value to a gemstone.
identified by gemologists, who describe gems and their
characteristics using technical terminology specific to the field of gemology.
The first characteristic a gemologist uses to identify a gemstone is
its chemical composition. For example, diamonds are made of carbon
(C) and rubies
of aluminium oxide (Al2O3). Next, many gems are crystals which are
classified by their 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.
classified into different groups, species, and varieties. For
example, ruby is the red variety of the species corundum, while any
other color of corundum is considered sapphire.
(green), aquamarine (blue), bixbite (red), goshenite (colorless),
heliodor (yellow), and morganite (pink) are all varieties of the
mineral species beryl.
characterized in terms of refractive index, dispersion, specific
gravity, hardness, cleavage, fracture, and luster. They may exhibit
pleochroism or double refraction. They may have luminescence and a
distinctive absorption spectrum.