Gem Stones and Birthston pietre naturale

7/29/2019 Gem Stones and Birthston pietre naturale

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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).

Gem Stones and Birthston pietre naturale

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