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E NATURAL GEMSTONES. HOWARETHEYFORMED?
,
be, fashioned to use for personal adornment. As a rule,
ones are beautiful, rare, and durable. Most are minerals:
, inorganic materials with a fixed chemical composition
ular internal structure. A few gems like Amber and
ome from plants and animals and are called organics.egarded as a gemstone, a mineral (or occasionally an
material) must be beautiful, most importantly in its
emstones must also be durable - hard enough to
constant use or handling without becoming scratched
aged. Finally, gemstones must be rare, because itsarcity endows it with a greater market value. The
ous appeal of gems, their exquisite colors, and the play
within them, would alone have made them precious to
Their rarity, hardness, and durability have made
nes doubly valuable. The natural beauty, strength, and
ce of gemstones have inspired beliefs in their
atural origins and magical powers, and gemstones that
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FACETING
The most usual method of fashioning a gem is to cut the surface into a number of flatfaces, known as facets. This gives the stone its final shape, or"cut". The craftsman, or
lapidary, who cuts the stone aims to show its best features, taking into account its
colour, clarity, and weight. He may, however, have to compromise to retain weight and
therefore value. There are several stages in the cutting of a gemstone, each of which
may be carried out by a different expert. In our example, a rough diamond crystal is
fashioned into a brilliant-cut. This is the most popular cut for this stone because it
maximizes the gem's naturally strong light dispersion. However because each stone is a
different shape, or has imperfections within it, or because retaining the weight is of
paramount importance, the cut in its ideal form (the "make") may not be possible.
Nevertheless, the essential aim is to make the diamond bright and "sparkling", showing
flashes of colour called fire. To this end, the size, number, and angles ofthe facets are
mathematically calculated. The rough crystal is sawn or cleaved to obtain a basic
workable piece, then turned on a lathe against another diamond to give it a round shape.
The facets are then cut and polished in stages, and the stone is given a final polish
before mounting.
Natural Gemstones
Rough Cut Grind
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A round diamond crystal is
selected for faceting.
The top is cut off, and
the stone rounded on
a lathe byanother
diamond.
The central facet the flat
table is ground first, then
the bezel facets.
Top & Bottom
More facets are put on in
groups and in sequence - thestar facets & upper girdle on
the crown & lower girdle
facets & the culet on the
pavilion.
Finishing
A "Brillianteer" then
adds a further 24facets girdle and 16
below.
Mounting
After a final polish, the
stone is mounted inprecious metal.
POLISHING
The shine given to the surface of a stone -either by rubbing it with grit or powder, or
against another stone - is its polish. Dark-coloured gemstones and those that are
translucent or opaque, for instance opal and turquoise, are often polished rather than
faceted, as are organic gems. They may be polished as beads or as flat pieces to be
used in inlay work, or cut en cabochon with a smooth, rounded surface and usually a
highly polished domed top and flat base. The polishing of stones, is accomplished by the
use of very finely powdered abrasives such as corundum powder, tripoli, pumice, putty
powder, etc. Each gem material requires special treatment to obtain the best results.
The polishing powder should not be quite as hard as the material to be polished, else itmay grind rather than polish. The material is used with water or oil to give it a creamy
consistency. It is backed by laps of different materials for different purposes. Thus, when
backed by a fairly hard metal even tripoli, although much softer, will polish sapphire. On
a lap of wood, tripoli would fail to polish hard materials, but would polish amethyst or
other quartz gem. A change of speed of the lap, too, changes the effect of the polishing
material. The polishing material has to be of a uniform size, preferably water floated or
oil floated, to give good results. The lap has to be kept flat and true and the stone must
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be properly held, or the flatness of the facets, upon which brilliancy depends in part, will
be destroyed during the polishing. The softer materials, such as opal, require treatment
more like that accorded cut glass, and soft abrasive powders, such as pumice, suffice to
polish them. Probably hardly two lapidaries would work exactly alike in their treatment of
precious stones, & each guards his secrets, yet all use approximately similar general
methods. Some have devised mechanical holders which permit the repeated cutting ofstones to exactly the same angles, and that, too, with an accurate knowledge of the
angles used. These angles can be definitely altered for different materials, according to
their refractive indices. Other lapidaries produce very fine results by purely hand
methods.
HOW ARE GEMSTONES FORMED
Most gemstones are mineral crystals that formed during one of three rock making
processes: igneous rocks, metamorphic rocks and sedimentary rocks. Gemstones that
have a mineral origin are found in rocks, or in gem gravels derived from these rocks.
Rocks them?selves are made up of one or more minerals, and may be divided into threemain types. The formation of these three types -igneous, sedimentary, or metamorphic - is
a continuous process, best described in terms of the rock cycle. Gem-quality minerals
with?in these rocks may be easily accessible at the Earth's surface, or lie buried deep
beneath it. Others, separated from their host rock by erosion, are carried by rivers to lakes
or the sea.
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IGNEOUS ROCKS
Deep within the earth's inferno is a stew of molten rock and
gases, called magma. As magma wells up within the earth,
intense pressure builds forcing the liquid rock toward the
surface, sometimes breaking through to be known as lava. It
slowly cools forming igneous rocks, and within these rocks
and gas bubble spaces, interlocking crystals grow. The
minerals present, the cooling time and the environment will
all play a role in the way these crystals form. Examples of
gemstones found in igneous rock: diamonds, topaz, kunzite
and spinel.
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METAMORPHIC ROCKS
Lastly, intense pressure and high temperatures can actually
recrystallize, transforming the composition of the rock. This
can happen deep within the earth's crust and it can also
happen with direct contact with hot magma. Emerald
sometimes also forms in hydrothermals associated withmagma. Other examples of metamorphic gemstones include
sapphire, jadeite and peridot.
SEDIMENTARY ROCKS
Rock fragments near the earth's surface are often washed
into riverbeds and seabeds. Over time, layers of rock
fragments, mud and other organic and non-organic elements
are compacted together in hard rock. Sometimes sea
creatures, bugs or other organic elements will get trapped
within the rock formations. Examples of sedimentary rocks
include limestone, shale and sandstone. In addition, most
gemstones are found in sedimentary deposits. Discovered in
riverbeds, these alluvial deposits include metamorphic gems
such as sapphire and ruby.
ORGANIC GEMS
Organic gems come from plants and animals. Natural pearls
form around foreign bodies that have made their way inside
the shells of marine or freshwater shellfish. Cultured pearls
are produced artificially in large fisheries, many in the
shallow waters off the shores of Japan and China. Shells
treated as gems may come from animals as diverse as snails
and turtles, living in the ocean, in fresh water, or on land.
Coral is made up of the skeletons of tiny marine animals
called coral polyps. Bone, or ivory from the teeth or tusks of
mammals, may come from recently living animals or from
fossils thousands of years old. Amber is fossilized tree resin,
collected from soft sediments or the sea. Jet is fossilized
wood, found in some sedimentary rocks.
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NATURAL INCLUSIONS
Most minerals contain visible traces of their genesis - perhaps tiny
crystals of other minerals that were caught up in the growth of the
larger host crystal, or formed simultaneously as it grew, internal
fractures that have been partially healed during growth, or traces
of earlier growth stages, marked by zoning. Gemologists use the
word inclusions to describe these and other internal
phenomena.
Inclusions in a gemstone, viewed through a microscope or a 10xloupe, can give information about the geological environment in
which the mineral was formed.
Inclusions can sometimes identify gemstones and they can prove
whether the stone is natural or synthetic. Inclusions can also tell
us where a gemstone comes from
Diamond with
Garnet
Solid inclusions may
be the same gem
type as the host, or
different?like the
garnet in this
diamond.
Inclusions can indicate about the geologic origin of the stone. They can tell you the storyof the gemstone formation: They are the natural trademark of the stone! The proof of its
natural, synthetic or treated status.
Insect in Amber
Inclusions are extremely precious to the gemologist as more
synthetics and treated stones are present in the market
everyday. Learning to recognize these inclusions is essential
knowledge for the gemstone buyer as he will be better able to
separate the fine natural gems from material that is the result of
human expertise in crystal growing or heat treatment.
Gemologists classify inclusions in 3 groups:
Protogenetic: Which were formed before the gemstone host, theyare always mineralistic, typically crystals more or less corroded.
Singenetic: That formed at the same time as the host crystal.
They were imprisoned inside it during its formation or have
appeared during its cooling as rutile needles in rubies and
Epigenetic: Formed after the gem was grown. Typical from this
type are limonite, iron staining or oil that can fill the natural
fissures of the gemstones.
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Emerald
Rectangular cavities with
tails are sometimes found
in Indian Emeralds.
Almandine Garnet
The bright
interference colours
at the bottom are due
to zircon crystals
Quartz
Needle like rutile
inclusions are
sometimes found in
Quartz
Moonstone
The insect like
inclusions are a
common feature of
Moonstone
Furthermore, some inclusions are just beautiful in themselves:
Golden needles in high luster quartz, insects trapped in Amber,
fine liquids of incredible complexity in sapphire, crystals looking
like star ships in rubies, cavities with tails in emeralds, or
sapphires. Some gemstones rely on their inclusions for their
effect, like aventurine quartz, or star sapphires. Green aventurine
quartz is actually colourless quartzite, filled with tiny flakes of
green fuchsite mica that lend it colour.Rutile Needles in
Rock Crystal
Star sapphires are filled with elongated crystals of rutile (often
called 'silk') arranged in three directions parallel to the faces of the
hexagonal prism. When such a stone is cut in a dome shape, with
its base parallel to the basal plane of the crystal, a six-pointed star
(or sometimes a twelve-pointed one) is seen in the reflected light
of the sun or a spotlight.
Star Sapphire
Inclusions are definitively no longer considered blemishes that
lower the value of gemstones. They are precious and beautifulwonders that unquestionably add value to them! The recognition
and the understanding of these inclusions is a key skill to learn for
all gemologists or gemstone buyers who want to know which
stones they are dealing with
Peridot
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Inclusions that look like
water lily leaves are a
typical feature of
peridot.
CHEMICAL PROCESS
Jewels are also the end result of a process begun billions of years ago when the Earth
was first formed. All that existed originally were the elements. Common elements you
may be familiar with include oxygen, silicon, and carbon. Gold and silver are also
elements. Far below the Earth's surface, where they were subjected to tremendous heat
and pressure, certain elements combined to form minerals.
Atoms of an element may combine with other atoms of the same element; or they maycombine with atoms from other elements to make compounds. Sulfur is a pure mineral,
that is, it contains only sulfur atoms. Sulphur atoms may also combine with iron atoms to
form a compound mineral called pyrite.
Diamond, which is pure carbon, is the only gemstone formed from one element. All the
other minerals that we classify as gemstones are compounds--they are minerals that
include atoms from more than one element.
Minerals formed from more than one element are also called chemicals. Like other
minerals, gemstones can be described by chemical formulas. For example, quartz is a
combination of silicon and oxygen called silicon dioxide.
Structure of a diamond
Arrangement of carbon atoms in a diamond gives it a
highly refractive surface which makes it shine
elegantly.
BRILLIANT-CUTS
The brilliant-cut is the most popular for diamonds, and for many other stones,
particularly colourless ones. It ensures that maximum light is reflected out
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through the front, giving brightness and fire. Variations in the outline give the
oval, the pear shaped pendeloque, and the boat-shaped marquise or navette.
STEP-CUTS
The step-cut (or (rap-cut) shows coloured stones to advantage, having arectangular or square table facet and girdle, with parallel rectangular facets.
The corners of fragile gems may be removed, making octagonal stones - as,
for example, in most emeralds.
MIXED-CUTS
Mixed-cut stones are usually rounded in outline, with the crowns (above the
girdle) cut as brilliants, and the pavilions (below the girdle) step-cut.
Sapphires, rubies, and most transparent coloured stones are cut in this style.
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FANCY-CUTS
These have several possible outlines, such as triangular, kite-shaped, lozenge-
shaped, pentagonal, ' hexagonal. The cut may be used for rare gems, or to
make the most of a flawed or irregularly shaped gem
CARVING
Carving usually refers to the cutting of
decorative objects from a larger mass. Stones as
hard as 7 on the Mohs' scale were carved in
Ancient Egypt, Babylonia, and China. Impure
corundum (emery) was used for carving and
engraving in India; nowadays a hand-held chisel
or turning machine is used. Popular stones for
carving include serpentine, Blue John, malachite,azurite, rhodonite, and rhodochrosite.
A frog carved out of an
emerald rock
A tortoise carved from black
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A face carved on a moonstone.
onyx and fashioned with
gold.
ENGRAVING
Engraving usually refers to the decoration of the
surface of a gemstone by the excavation
(scratching out) of lines, holes, or trenches with
a sharp instrument, known as a graver or turin.
Of all engraved objects, cameos and intaglios
are perhaps the most popular. A cameo is a
design (often a human profile) in Hat relief,
around which the background has been cut
away. In an intaglio it is the subject, not the
background, that is cut away, creating a
negative image that may be used as a seal in
clay or wax.Intaglios were particularly popular with the
Ancient Greeks and Romans, and are still prized
by collectors. Engraved gemstones gained
prominence in Europe in the Renaissance period.
During the Elizabethan period in Britain cameo
portraits were often given as gifts, particularly
among the nobility. All through the ages, layered
stones have been used for cameos or intaglios,
with onyx and sardonyx particularly popular.
Other gems suitable for engraving include rockcrystal, amethyst, citrine, beryl, peridot, garnet,
lapis lazuli, and hematite, as well as organic
materials such as ivory and jet
IMITATION
Imitation gems have the appearance of their natural counterparts, but their physical
properties are different. They are made to deceive. Man-made materials, such as glass
and synthetic spinel, have been used to imitate many different gems, but natural stones
can also be modified to resemble more valuable gems. It is possible to enhance
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authentic gemstones by hiding cracks and flaws, or by heat-treating or irradiating to
improve their colour.
GLASS IMITATIONS
Glass has been used for centuries to imitate gemstones. Itcan be made either transparent or opaque, in almost any
colour, and, like many gems, has a vitreous lustre. At first
sight, therefore, it may easily be mistaken for the real
thing. However, it can usually be detected by its warmer
feel, and by the evidence of wear and tear that results
from its greater softness. Chipped facets and internal
swirls and bubbles are common. In addition, unlike most
of the gems it imitates, glass is singly refractive.
Glass Ruby
HEAT TREATMENT
Heating may enhance or change the colour or clarity of
some gems. Techniques range from throwing gems in a
fire to "cook", to use of sophisticated equipment. The
outcome is certain for some gems (like aquamarine,
changing from green to blue), but less so for others. Yellow topazheated to Blue
STAINING
Stains, dyes, or chemicals can alter the appearance of a
gem, coating just the surface, or changing the whole
specimen. For staining to be effective, a stone must be
porous or contain cracks and flaws through which the
colour can enter. Porous white howlite, for example, can
be stained imitate turquoise.
Stained howlite
IRRADIATION
Gems may change colour if exposed to radiation. This may come fromradioactive elements within the Earth's crust, or from artificial sources.
Natural radiation may take millions of years to have an effect, while artificial
irradiation may take only a few hours to change a gem's colour. In some cases
a gem will revert to its original colour, or may fade with time. Many changes
can be reversed or modified by heat treatment.
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OILING
Oils may enhance a gem's colour and disguise fissures
and blemishes. It is common to oil emeralds in order to fill
their natural cracks and flaws.
Oiled emerald
LUSTRE
The luster or brilliance of transparent gems is caused by light reflecting from the stone's
surface. The smoother and more highly polished the surface is, the greater the luster will
be. High light refractivity of a gem will cause greater luster as well. The most intensive
luster is seen in the highest refractive indices, diamond, zircon, and rutile, and is known
as an adamantine luster. Hematite produces a metallic luster, even though it is not
transparent. Most gemstones have a vitreous or glassy luster, but there are other types
of lusters, including resinous (amber), greasy (serpentine), waxy (turquoise), pearly
(rhodonite), and silky (tiger's eye) The overall appearance of a gemstone, its "lustre", is
determined by the way light is reflected from its surface. Gemmologists use a variety of
terms to describe lustre and its degree of intensity. "Splendent" means that the stone
reflects light like a mirror; but if little light is reflected, the lustre may be described as
"earthy" or "dull". Stones with a lustre comparable to diamond are described as"adamantine", and are the most desirable. In fact, most transparent, faceted gems have
a glass-like, "vitreous" lustre; the precious metals have a "metallic" lustre; and organic
gems show a range, from "resinous" to "pearly" and "waxy". Some gemstone species
vary in their lustre: garnets, for example, range from the resinous hessonite garnet to the
adamantine lustre of demantoid garnet. Rough lazulite and howlite have a dull, earthy
lustre, which is vitreous after polishing
INTERFERENCE
Interference is an optical property caused by the reflection of light off structures within a
gemstone. This internal reflec?tion gives a play of colour. In some stones it will produce
the full range of the spectral colours; in others just one colour may predominate. In opal,
interference occurs because of the structure of the stone itself?spheres arranged in
regular three-dimensional patterns. This produces the rainbow effect called iridescence,
also shown by a number of other gems such as hematite, labradorite, and iris quartz. In
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moonstone feldspar, interference at the junctions of its internal layers (thin, alternating
layers of different types of feldspar) produces a shimmering effect just below the surface
of the stone, known as adular-escence, opalescence, or a schiller (sheen).
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