organic materials, such as pearl, amber,
jet, and coral. Customarily, diamond,
ruby, sapphire, and emerald are the
major gems.
The most important qualities of
gemstones   are   beauty,  durability,
uniqueness, and rarity. Beauty, indicated
as splendor, purity, or attractiveness, is
according to the taste of the beholder and
includes such appearances as luster,
transparency, brilliance, and color.
Luster of a mineral or stone is
independent of color and is the surface
appearance m reflected light. Apart from
materials that have a metallic luster, the
chief  contributors  to  luster   are
transparency and refractive index. In cut
gems, the perfection of the polish
enhances    the  luster.      Visible
imperfections impair the luster of
transparent  stones.   Yet,   defects,
described as "jardens" or "inclusions,"
may enhance the beauty and value of
natural rubies, emeralds, and other
gemstones. Sometimes these inclusions
may be used to identify the country and
even the mine from which the stone
came. Durability is the resistance of a
stone to abrasion, pitting, chipping, or
splitting.  Resistance to abrasion is
correlated with relative hardness, but
intrinsic brittleness and toughness suggest
resistance to wear in other aspects.
Rarity is an essential qualification and is
more important for some stones in
determining their value than their physical
characteristics.
Of the approximate 2,900 mineral
species, only about 100 possess all the
attributes required of a gem. Collectors
of gems may not require that a gem be
durable because the stone is for display
and is not to be worn. Therefore, the
number of species of gemstones may be
greater than the 100 that meet all the
requirements.
Silicates furnish the greatest number,
including such minerals as beryl, topaz,
tourmaline, and feldspar. Oxides such as
corundum (ruby and sapphire) and quartz
(amethyst, agate, etc.) comprise the
second  largest  group.      Sulfides,
carbonates, and sulfates are of small
importance; the phosphates yield
primarily turquoise and variscite. An


exception is pearl, essentially calcium
carbonate, which ranks high as a gem.
Diamond, the best known gem, is an
isometric crystalline form of the element
carbon.
Gemstones are classified the same as
minerals; that is, into group, species, and
variety. Group refers to two or more
gem materials that are similar in crystal
structure and physical properties but have
different chemical properties.  Each
member of the group is a species.
Varieties of species have similar crystal
structure and chemical characteristics but
differ in color. An example of this
would be the hessonite variety of
grossular species of the garnet group.
Products for Trade and Industry
Cutting of gems from gemstones is to
obtain the most effective display of the
material. No significant change is made
in the fundamental properties, and the
preparation is to enhance the desirable
characteristics that are present initially.
Gemstones are cut into gems in three
main styles: faceted, cabochons, and
baroque.
Facet cutting usually is on transparent
gemstones to increase brilliancy and
appearance. Often it is confined to the
harder materials. Softer materials may
be faceted, but extreme care must be
exercised in cutting and polishing the
stones and in their use in jewelry. Often
the softer gems are only for display and
not for making jewelry. The "round
brilliant" cut, most commonly used in
faceting, has 58 facets, 33 above the
circle "girdle" and 25 below it, arranged
in eightfold symmetry.   The "round
brilliant" and other common cuts are
illustrated in figure 1. (See figure 1.)
Cabochons are cut in four operations:
sawing, grinding, sanding, and polishing.
Sawing, the first step in cutting,
customarily is done with a diamond saw
to obtain a slab or slice of the desired
size and thickness from the rough
gemstone.   The cabochon outline is
scribed onto a flat surface, using a
template for making a standard size for
jewelry mountings. Rough grinding of
the stone may be by metal-bond diamond,


electroplated diamond, silicon carbide, or
aluminum oxide wheels or coated
abrasive disks. In grinding, the hardness
of the gemstone determines the grit and
hardness of the abrasive used. Multiple
grinding steps starting with 80- to
100-mesh (grit) through 600-mesh
abrasives are used. The scratches left by
grinding are removed by progressively
finer grinding and sanding. Disk or belt
sanders use abrasives bonded to cloth,
waterproof reinforced paper abrasives, or
cloth charged with abrasive pastes. The
final polish is on hard felt, wood, or
leather laps, with various polishing agents
such as fine diamond compound, tin
oxide, tripoli, chromium oxide, cerium
oxide, alumina, or rouge.
Polished irregular shapes are baroque
gems.    An  inexpensive method of
polishing baroque gems is to tumble them
in rubber-lined drums, using a grinding
and polishing medium with or without
water.
Industry Structure
The Central Sales Organization (CSO),
the marketing arm of De Beers Centenary
AG, highly controls the world market for
rough diamonds. It is by far the most
controlled of the world's commodity
markets. The CSO markets about 80%
of the world's gem and natural industrial
diamond.   The marketing through the
CSO is by the Diamond Trading Co. Ltd.
and Industrial Distributors Ltd. The CSO
sells uncut gem diamonds for De Beers
and most other major producers at sights
(approved bidder viewings) in London,
England, and Lucerne, Switzerland.
There are 10 such sights each year.
Diamonds reach the CSO sights
through three channels-De Beers owned
and operated mines, contracts sales by
mine owner and operators, and open-
market competitive sales.
The distribution of rough diamonds in
the Republic of South Africa is by the
South Africa Diamond Board. A new
agreement was reached between De Beers
Consolidated Mines Ltd., its customers,
and the Government of South Africa on
the method of domestic rough diamond
distribution. In the past, all categories of


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