GEMSTONE : CRYSTAL SHAPES

Most mineral gemstone are crystalline, with their atoms arranged in regular and symmetrical patterns, like a lattice;a few are amorphous, with no or only a weak crystal structure. Crystalline minerals may consist of a single crystal, or of many in a group. Polycrystalline minerals are made up of many, usually small, crystals; in cryptocrystalline minerals the crystals are too small to see without the aid of a microscope.

Crystalline minerals are made up of a number of flat surfaces called faces; the orientation of these faces defines the overall shape, which is known as the "habit". Some minerals have a single. characteristic habit, such as pyramidal or prismatic; others may have several. A lump of crystalline mineral without a definite habit is called massive. Amorphous gemstones, like obsidian and tektites, have an irregular shape.

TWINNING

Natural crystals are seldom perfect. Their growth is influenced by external factors such as temperature, pressure, space, and the medium in which they grow. One irregularity that may occur is known as "twinning" - when the internal structure of the crystal is repeated. Twins grow together in a number of different ways and may show alternative colours.

GEMSTONES : PHYSICAL PROPERTIES

The physical properties of gemstones, their hardness, their specific gravity or density, and the way they break or "cleave", depend on chemical bonding and the atomic structure within the stone. For example, diamond is the hardest natural material known, and graphite is one of the softest, yet both are made of the same element, carbon. It is the way in which the carbon atoms are bonded together in diamond that gives it a reater hardness and resilience.

HARDNESS

One of the key qualities of a gemstone, hardness maybe measured by how well a stone resist scratching. Every stone can be tested and classified using the Moh's scale of hardness, which gives every mineral a figure from one to ten. Intervals between numbers on the scale are not equal, most obviously between nine and ten. Hardness testing is destructive, however, and should only be used on a gemstone if other tests fail.

SPECIFIC GRAVITY

The spesific gravity (SG) of a gem is an indication of its density. It is calculated by comparing the stone's weight with the weight of an equal volume of water. The greater a stone's specific gravity, the heavier it will feel. For example, a ruby with an SG 4.00 will feel heavier than an emerald with an SG 2.71 of similar size.

CLEAVAGE AND FRACTURE

Gemstones may break in two ways : they either cleave or they fracture. Which way they break depends on the internal atomic structure of the stone. Gems that cleave tend to break along planes of weak atomic bonding (cleavage planes). These planes are usually parallel, perpendicular, or diagonal to the crystal faces (as both planes and faces are directly related to the stone's atomic structure). A gemstone may have one or more directions of cleavage, which may be defined as perfect (almost perfectly smooth), distinct, or indistinct. Gems with perfect cleavage include diamond, fluorite, spodumene, topaz, and calcite.

When a gemstone breaks along a surface that is not related to its internal atomic structure, it is said to fracture.

HOW GEMSTONES ARE FORMED

Gemstones that have a mineral origin are found in rocks, or in gem gravels derived from these rocks.Rock themselves are made up of one or more minerals, and maybe divided into three main types. The formation of these three types - igneous, sedimentary, or metamorphic - is a continuous process. Gem quality minerals within this 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 sea.

IGNESIOUS ROCKS
Igneous rocks have solidified from molten rock, which comes from deep beneath the Earth's surface. Some, called "extrusive" igneous rock, are thrown out from volcanoes as lava, volcanic bombs, or ash. "Intrusive" igneous rocks are those that solidify beneath the surface. Essentially, the slower a rock cools and solidifies, the larger the crystals - and therefore the gemstones - formed within it. Many large gemstone crystals form in a kind of intrusive igneous rock known as pegmatite.

SEDIMENTARY ROCKS
Sedimentary rocks are formed by the accumulation of rock fragments produced by weathering. In time, these fragmentssettle down and harden into rock once more. Sedimentary rocks are usually laid down in layers, and these may be shown as a feature in decorative stones. Most Australian opal occurs in sedimentary rocks; turquoise occurs mainly as veins in sedimentary rocks such as shale; halite and gysum are sedimentary rocks.

METAMORPHIC ROCKS
Metamorphic rocks are either igneous or sedimentary rocks that have been changed by heat and pressure within the Earth to form new rocks with new minerals. As this happens, gemstones can grow within them. Garnets, for example, form in rocks called mica schist, which were once mudstones and clays. Marble, formed from limestone that has been subjected to intense pressure and high temperatures, may contain ruby.

ORGANIC GEMS
Organic gems come from plants and animals. Natural pearls from around foreign bodies that have made their way inside the shells of marine or freshwater shellfish. Cultured pearls are produced artificially in large fisheries, may in the shallow waters off the shores of Japan and China. Shells treated as gems may come from animals are diverse as snails and turtles, living in the ocean, in fresh water, or on land. Coral is made up of the skeletons of tiny marineanimals called coral polyps. Bone, or ivory from the teeth and 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.

WHERE GEMSTONES ARE FOUND

Some gem minerals, such as quartz and garnet, are found worldwide. Others, like diamonds and emeralds are rarer, due to the more unusual geological conditions necessary for their formation. Even when a mineral is found worldwide, only a minute proportion may be of gem quality. The main gem localities of the world are therefore those where gem quality material occurs in sufficient quantity to make production economic.

DIAMOND
Diamond forms at high temperatures and pressures 80km ( 50 miles ) or more underground. When India and later Brazil were the main producers, most diamond came from secondary sources, such as rever gavels. How ever, since the discovery of diamond in kimberlite rock in South Africa ( around 1870 ), its extraction has involved processing vast quantities of rock. Australia is the main producer today, other localities include Ghana, Sierra Leone, Zaire, Botswana, Namibia, the former USSR, the USA and Brazil.

EMERALD
Emerald found in granites, pegmatites, and schists, as well as alluvial deposits, the finest emeralds are from Colombia. Other sources are Austria, India, Australia, Brazil, South Africa, Egypt, the USA, Norway, Pakistan and Zimbabwe.

RUBY ( CORUNDUM )
Ruby found in worldwide, in igneous and metamorphic rocks, or as waterworn pebbles in alluvial deposits. The finest stones come from Burma; those from Thailand, the main source are brownish red; Afghanistan, Pakistan, and Vietnam produce bright red stones; those from India, North Carolina (USA), Rusia, Australia and Norway are dark, sometimes even opaque.

SAPPHIRE (CORUNDUM)
Good quality sapphire is found in Burma, Sri Lanka, and India. The best Indian sapphire is cornflower blue, and found in Kashmir, either in pegmatites or as waterworn pebbles in alluvial deposits. Sapphire from Thailand, australia, and Nigeria is dark blue,and may appear nearly black. Montana (USA) produces sapphire of an attractive metallic blue. Other lacalities iclude Cambodia, Brazil, Kenya, Malawi,and Colombia.

PEARL
Natural pearl have been harvested from the Persian Gulf, the Gulf of Manaar (Indian Ocean), and the Red Sea for thousands of years. The coasts of Polynesia and Australia produce mainly cultured pearl. Both freshwater and saltwater pearls are cultivated in Japan and China. Freshwater pearls occur in the rivers of Scotland, Ireland, France, Austria, Germany, and Mississippi (USA).

FIVE MAJOR GEMSTONES : These Five Stones are the Most Highly Prized

What is a gemstone? To be regarded as a gemstone, a mineral ( or occasionally an organic material ) must be beautiful, most importantly in its colour. A gemstones also be durable - hard enough to survive constant use or handling without becoming scratched or damaged. Finally, it must be rare, because its very scarcity endows it with a greater market value.

There are over 3,000 different minerals, but only about 50 are commonly used as gemstones. Others are cut for collectors of the unusual, but are often not suitable for wear because they are too soft and easily scratched. In fact, the number of minerals regarded as gemstones constantly changes, as new sources and varieties are found and fashions changed.

These five gemstones are the most highly prized : diamond, sapphire, ruby, emerald and pearl. All except pearl have a particular "cut" that brings out their best quality.

DIAMOND

Diamond is the hardest mineral on Earth, and this, combined with its exceptional lustre and brilliant fire, has made it the most highly prized of all gems. Pure, colourless diamond is the most popular, but other varieties - from yellow and brown to green, blue, pink, red, grey, and black - are also found, depending on the impurities present.

Because of the uniform arrangement of their constituent carbon atoms, diamond crytals are well-formed - usually octahedral with rounded edges and slightly convex faces.

Their perfect cleavage facilitates the early stages of fashioning, but they can only be polished by other diamonds.

EMERALD ( BERYL )

Emerald derives its beautiful green colour from the presence of chromium and vanadium. Emeralds are rarely flawless, so stones are often oiled to fill ang disguise cracks, hide flaws, and enhance colour.

To minimize the loss of material, the step-cut ( or "emerald cut", as its known ) is commonly used, but ancient engravings are known, and cameos, intaglios, and beads can make the best of a flawed stone.

RUBY ( CORUNDUM )

Ruby - the name given to red, gem-qualiti corundum - is one of the best gemstones for jewellery settings. Rubies may be any shade of red, from pinkish to purplish or brownish red, depending on the chromium and iron content of the stone.

Frequent twinning of the crystals makes the material liable to fracture, yet ruby is a tough mineral, second only to diamond in hardness. Crystals prisms are hexagonal with tapering or flat ends. As the crystals grow, they form new layer and depending on the geological conditions and minerals present, colour variations, called zoning, occur.

SAPPHIRE ( CORONDUM )

All gem-quality corundum that is not red is called sapphire, yet this name is popularly associated with the colour blue. Variation in colour, due to iron and titanium impurities, spans many shades, but the most valuable is a clear, deep blue.

Some stones, called "colour-change sapphire", exhibit different shades of blue in artificial and natural light.

PEARL

Pearl are formed in shellfish - especially oysters and mussels - as a natural defence against an irritant, such a piece of grit. Layers of aragonite, known as nacre, are secreted around the irritant, and gradually build up to form the solid pearl. Light reflecting from these overlapping layers produces a characteristic iridescent lustre, also known as the "orient of pearl".

In cultured pearls an irritant is introduced to initiate the formation of a pearl. In a "nucleated" cultured, pearl a small bead is used as the nucleus, upon which the layers of nacre are secreted.

Pearls vary in colour, from white, white with a hint of colour ( often pink ), to brown, or black, depending on the type of mollusc and the water. They are sensitive to acids, dryness, and humidity, and so are less durable than many other gems.