Gemstone - What is it and how is it formed?

Gemstone: The mysterious appeal of gemstones, their exquisite colors, and the play of light within them, would alone have made them precious to many. However, their rarity, hardness, and durability have made them doubly valuable. The natural beauty, strength, and resilience of gems have inspired beliefs in their supernatural origins and magical powers. Also, the stones that have survived the centuries have gathered a wealth of history and romance around them.

There are over 3,000 different minerals. Yet, only about 50 are commonly used as gemstones. Others are cut for collectors of the unusual. However, they 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 fashion changes.

Over 130 gem species are exceptionally rare stones. These illustrate the very wide range of naturally occurring gemstones. When cut, they epitomize the allure of gemstones, i.e, beauty, durability, and rarity.

What is a Gemstone?

A gemstone is a mineral that has been or may be, fashioned to use for personal adornment. As a rule, it is beautiful, rare, and durable. Most gemstones are minerals: natural, inorganic materials with fixed chemical composition and regular internal structure.

To be regarded as a gemstone, a mineral (or occasionally an organic material) must be beautiful, most importantly in its color. There are five prominent gemstones that are the most highly prized. These are diamond, ruby, emerald, sapphire, and pearl.

A gemstone must also be durable, i.e., hard enough to survive constant use or handling without becoming scratched or damaged. Finally, it must be rare, because its scarcity endows it with a greater market value.

A gemstone is a mineral that has been or may be, fashioned to use for personal adornment. As a rule, they are beautiful, rare, and durable. Most are minerals: natural, inorganic materials with fixed chemical composition and regular internal structure. A few gems, like amber and pearl, come from plants or animals and are known as organics. Others, called synthetics, do not have a natural origin but are made in laboratories.

They have very similar physical properties to natural gems and may be cut to imitate the real stone. The rough crystals may be rounded and polished naturally or ground mechanically. In its natural state, the mineral may be a prism with clearly defined facets.

How a gemstone is formed?

A Gemstone that has a mineral origin is found in rocks, or in gem gravels derived from these rocks. Rocks themselves are made up of one or more minerals and may be divided into three main types. The formation of these three types – igneous, sedimentary, or metamorphic – is a continuous process. It is best described in terms of the rock cycle.

Gem-quality minerals within these rocks may be easily accessible at the Earth’s surface. These may also lie buried deep beneath it. Others, separated from their host rock by erosion, are carried by rivers to lakes or the sea.

1. Metamorphic Rocks

Metamorphic rocks may be either sedimentary or igneous rocks whose character is fundamentally 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 schists, which were once mudstones and clays. Marble, formed from limestone that has been subjected to intense pressure and high temperatures, may contain rubies.

2. Igneous Rocks

Igneous rocks have solidified from molten rock, which comes from deep beneath the Earth’s surface. Some, called extrusive igneous rocks, 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. Peridot crystals form as lava cools in certain locations.

3. Sedimentary Rocks

Sedimentary rocks are formed from the accumulation and compression of eroded rock fragments. They may eventually be buried back below the surface. In time, these fragments settle down and harden into a 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 gypsum are sedimentary rocks.

How a Gemstone is Mined?

For many, the real satisfaction comes from actually owning gems. Most people cannot afford the more expensive stones. However, anybody can collect a few minerals that, even if not gem-quality, are still very attractive. You may even chance on a piece of amber on a beach, or come across a beautiful piece of jewelry in a local auction.

No matter how modest your collection, it will give you hours of fascination and enjoyment.

In the 18^th century, jeweled trinkets on a decorative box were fashionable.

Modern Mining

Some gemstones are so valuable that large-scale mining, in which tons of rock may be extracted to collect tiny amounts of a gemstone, is still viable.

Alluvial Mining

Small-scale mining with traditional methods and equipment, such as in Sierra Leone, is still common in many under-developed countries.

Fossicking (Foraging)

Fossicking (foraging) for opals and sapphires is possible in parts of Australia. However, it is mandatory to first obtain a permit from the authorities. River beds and streams are the best locations. In many parts of the world, traditional methods and equipment are still used for collecting gemstones.

Where is a gemstone found?

Some gemstone minerals, such as garnet and quartz, are found worldwide. Others, like emeralds and diamonds, 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 gemstone localities of the world are therefore those where gem-quality material occurs in sufficient quantity to make production economic.

1. Diamonds of Africa

The kimberlite rocks of southern Africa are mined in a modern, large-scale way, producing vast quantities of diamonds for both industrial and gem use.

2. Pearls in Japan

The shallow coastal waters of the Japanese islands offer ideal conditions for farming pearl oysters. Pearls are organic gems, and therefore independent of geological conditions.

3. Rubies and Sapphires in Myanmar (Burma)

The rich mineral deposits of Mogok in Myanmar (Burma) have yielded some of the world’s finest rubies, although extraction is by traditional methods. Sapphires are also mined there.

The Science of A Gemstone

Gems are scientifically fascinating, too. Gemologists make a complete study of each stone, both as it is found in rocks and after it has been cut and polished. The internal structure of a stone is observable using magnification via a microscope. In this world within a world of gemologists may turn detectives, being able to distinguish between two outwardly similar stones, or between a natural and a fake stone.

Physical Properties of a Gemstone

The physical properties of gemstones, their hardness, specific gravity or density, and the way they break or ‘cleave,’ depend on chemical bonding and the atomic structure within the stone. For instance, 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 a diamond that gives it greater hardness and resilience.

1. Hardness

One of the key qualities of a gemstone, hardness may be measured by how well a stone resists scratching. Every stone can be tested and classified using the Mohs’ scale of hardness. It 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.

Knoop Scale

The Knoop scale shows the indentation caused by a diamond point when it meets the surface of a mineral. The 10 stages correspond to Mohs’ points.

Mohs’ Scale of Hardness

The Mohs’ scale was devised by the German mineralogist Friedrich Mohs as a means of classifying the relative hardness of minerals. He took ten common minerals and put them in order of scratchability. Each gem will scratch those below it on the scale but will be scratched by those above.

1 – Talc
2 – Gypsum
3 – Calcite
4 – Fluorite
5 – Apatite
6 – Orthoclase
7 – Quartz
8 – Topaz
9 – Corundum
10 – Diamond

2. Specific Gravity

The specific 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 instance, a small cube of pyrite, with an SG of 5.2, will feel heavier than a larger piece of fluorite with an SG of 3.18; and a ruby (SG 4.00) will feel heavier than an emerald of similar size.

3. Cleavage

Gemstones may break in two ways: they either cleave or 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 places 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. Similarly, fragile barite has three directions of easy cleavage, giving smooth surfaces. On the other hand, the cleavage direction is indistinct in aquamarine.

Albite has a distinct cleavage even though it is not perfectly smooth.

4. Fracture

When a gemstone breaks along a surface that is not related to its internal atomic structure, it is said to fracture. Fracture surfaces are generally uneven, and each type has its own descriptive name.

An uneven fracture surface is typical of fine-grained or massive gems like dumortierite.

In nephrite, the splintery fracture is due to interlocking texture.

A hackly fracture is a rough, uneven fracture surface that is usually visible on the surface of gold.

The most common type of fracture in gemstones is called conchoidal. It refers to the shell-like fracture surface in gemstones like obsidian.

Organic Gemstone

A gemstone produced by living organisms is called ‘organic.’ Their sources are as diverse as shellfish (which produce pearls), polyps (whose skeletal remains form coral), and the fossilized resin of trees (which make amber). Ivory, jet, and shell are also organics. These materials are not stones and are not as durable as mineral gems. Instead of being cut in facets like mineral gems, they are usually polished or carved or drilled as beads.

An Organic gemstone comes from plants and animals. Natural pearls form around foreign bodies that have made their way inside the shells or freshwater shellfish. Cultured pearls are produced artificially in large fisheries, many in the shallow waters off the shores of China and Japan. Shells treated as gems may come from animals as diverse as snails and turtles, living in the ocean, in freshwater, 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 resin, collected from soft sediments or the sea. Jet is fossilized wood, found in some sedimentary rocks.

Synthetic Gemstone

A synthetic gemstone does not have a natural origin. So, scientists make it in laboratories. It has very similar physical properties to a natural gemstone and lapidary artists may cut them to imitate the real thing.

Man-made synthetic stones are similar in chemical composition and optical properties to their natural equivalents. In the flux method, scientists grow crystals, and the lapidarist facets them.

So, scientists make them in laboratories. They have very similar physical properties to natural gems and lapidary artists may cut them to imitate the real thing.

Man-made synthetic stones are similar in chemical composition and optical properties to their natural equivalents. In the flux method, scientists grow crystals, and the lapidarist facets them.

Imitation Gemstone

Gemstones have been imitated throughout the ages. Many lesser stones have been used, as well as glass paste and other man-made materials. Composite stones, like the red garnet on green glass, are made of more than one piece.

Polished Gemstone

Crystals may be naturally round or polished. They reach this shape due to the water flow in a stream or river. Similarly, lapidarist can ground them mechanically.

Faceted Gemstone

It is possible to cut a gemstone to give it a number of surfaces, called facets. The facets absorb and reflect light, to a magical effect.

Almost all cut gemstones begin their lives in a crystalline form. These are embedded in a host rock known as the matrix. In this state, the stone is “rough.” Many natural crystals are attractive enough and many collectors display them as they find them in nature. Others are available for faceting and polishing to enhance their beauty. The jeweler then sets them in a piece of jewelry or an ornament.

Cabochon

A simple fashion for stones is to cut them en cabochon. It is a domed and highly polished surface.

Precious Metals

The precious metals are platinum, gold, and silver. They are not true gemstones, but they are attractive and easily worked, often as settings for gems. So, they have their own intrinsic value. Platinum is the rarest and the most valuable.

Jewelry

A piece of jewelry, usually one or several polished or a faceted gemstone set in a mount of precious metal, is often the finished product.

Private Gemstone Collection

The Mathews gemstone collection in London comprises 4 boxes of un-mounted gems from all around the world and a group of Colombian emeralds. The scope of the collection is unique, but many fine examples of cut and uncut gemstones are on public displays in museums.

Kings and Commoners

Throughout the ages, gemstones have been seen as representations of power and wealth. Symbols of supremacy, from crowns to richly decorated robes, have traditionally been adorned with jewels.

However, gemstones are not just for the wealthy or scientific people. They can be loved by anyone, from the amateur gem-spotter to the enthusiast who enjoys their beauty and history.