Category Geology

There are many ways of shaping, or ‘cutting’, a gemstone. A person who cuts gems is called a ‘lapidary’. The gems are cut to display their colour, fire, sheen or other beautiful optical effect. Gems are sliced with diamond saws and ground into flat facets or curved surfaces. They are then polished with diamond or ruby powder.

Many clear gems are cut with flat, mirror-like facets. Their angles are carefully set to allow for the way that the light ‘bends’ as it enters and leaves the gem. Each kind of gem has its own special set of facet angles: brilliant cut ruby has different angles from topaz, for example. A faceted gem will twinkle or show its colour well only if it has been cut with correct facet angles.

Different types of cut: (a) emerald, (b) pear, (c) round, (d) oval, (e) marquise.

‘Cabochons’ are gems that have been cut in the shape of a dome. This type of cut shows off bright colours in opaque gems — gems that do not let light through. Cabochons are also made to reveal beautiful tricks of light, such as ‘stars’ in certain rubies and sapphires, the sheen in moonstones, colours in opals and the bright line which can be seen inside the rare, honey-coloured ‘cat’s eye’ gems.

The ‘emerald cut’ is oblong with the corners cut off. Long facets reflect lots of light back from deeply coloured, transparent gems. Some gems are carved so that little scenes, symbols or figure-heads stand out. These are called ‘cameos’. Opals are often cut into cabochons to show off their colours.

A diamond can be cut and shaped only by another diamond. This is because diamond is the hardest substance known to man. Each diamond crystal can only be cut in certain directions, along which it is a little less hard. Even so, it takes hours to saw through a diamond. Diamonds can also be split, or ‘cleaved’, along four different directions through the crystal.

Nearly all diamonds are ‘brilliant cut’. This means that the facets — faces of gems — are cut at just the correct angles to make the most of a diamond’s sparkle. Each facet acts like a polished mirror inside the gem — it reflects the light and splits it into the colours of the rainbow.

There are several different stages involved in shaping a diamond crystal into a cut gem. Firstly, the crystals are sent to special factories and sawed with thin bronze discs coated in diamond dust and olive oil. The designer decides where each crystal is to be sawed.

Each diamond is then ‘bruted, or shaped. The bruter shapes the gem by holding another diamond against it while it is spun around at great speed. After grinding and smoothing the top facet, or ‘table’, the cutter carefully decides where to grind the first of the 16 main facets.

When the main facets are polished to the right size, the ‘brillianteer’ grinds the other 40 small facets. Over half of the original crystal has now been cut or ground away!

Picture Credit : Google

Diamonds from the mine are sorted into two groups, industrial and gem quality. Many of the stones are full of bits of mineral which reduce their value and make them unsuitable for jewellery. Gem quality diamonds are sorted according to their weight, colour, clarity and shape. Every year over 20 tonnes of diamonds are mined, but only a small amount is of gem quality. The rest are industrial diamonds, and the demand for them is high.

Diamonds are probably one of the most abundant gems on Earth but they are hard to reach. The world’s supply and prices of diamonds are very carefully controlled. This has resulted in a huge inflation in the price of diamonds.

Most diamonds come from very big and deep mines. Often the mines are in the underground ‘pipes’ of extinct volcanoes. Firstly, the top rocks are removed to make a pit. Next, huge shafts are driven beneath the pit to reach the diamonds. On the south-west coast of Africa, another type of diamond mining takes place. There, diamonds lie buried in an ancient pebble beach now covered by huge sand dunes. Over 70 million tonnes of sand and pebbles have to be removed to extract half a tonne of diamonds.

Gems like opal, topaz and emerald mostly come from very small tunnels or gravel pits close to the Earth’s surface. The earth is scooped out of the pit, washed and sieved, and any gems are hand-picked from the sieve.

Some mines are extremely deep. You can see one of the mining levels in big diamond mine. Huge pits are cut out of the solid rock so that shattered rock falls through onto railway trucks running through a tunnel. Australian opal miners actually live inside mines. It takes a lot of work to free the gems from very hard rock. People also search the rock waste dumps hoping to find opals the miners may have overlooked!

Separating diamonds

Around one gramme of diamond crystals is taken from an average of about 22 tonnes of rock — the same weight as five adult elephants. It is only because diamond has special properties that such a tiny amount can even be detected and removed.

Nearly all the diamonds that are recovered are less than 3 cm across but even 1/2 cm crystals are ‘caught’ by the grease on the conveyor belt. Diamonds can also be spotted because they glow in X-rays.

Diamonds are cleaned and sent to the sorting room, then weighed and locked up.

Gemstones are made from rare crystals that have been cut up and polished. They are usually hard and clear, or colourful. Gemstones are extremely valuable because of their beauty and rarity. Usually when we think of gemstones we imagine a jeweller’s shop window, or a beautiful ring or necklace. Polished gemstones are set into jewellery and decorative objects.

Hard or clear crystals are not only used to make jewellery, they are also used in factories, spacecraft and lasers.

Natural gemstones are found in the Earth. Crystals in the Earth’s rocks are called minerals. Gems made from these minerals are called natural gemstones. Artificial gemstones are made from glass and from crystals made in laboratories and factories. Gems of all kinds are often made to imitate more valuable gemstones.

Picture Credit : Google

The Earth’s rocks are moving, squashing and often melting together. It is in this turmoil that gems are formed. The rocks are a mixture of minerals which are themselves made up of chemical elements. Atoms of chemical elements, such as carbon, oxygen and silicon, join up to make tiny building blocks. These fit together in neat regular patterns to form ‘crystals’.

Exactly what type and size of crystal forms depends on the chemical elements present, the temperature of them and the pressure exerted on them. Natural gems are found as clear or coloured crystals embedded in rock. They are also found as big crystals lining cracks or cavities in the Earth’s surface layer, or ‘crust’. Some gems are washed along by rivers and may then be picked out.

How gems are formed

Some gem crystals, like garnet, grow in solid rock as it squashes beneath moving mountains. Others, such as tourmaline, are formed in veins beneath the Earth’s surface.

Stones such as agate grow in volcanic rocks at the surface. Gems like ruby grow deep down where molten rock ‘cooks’ the Earth’s crust. Diamond crystals grow way down, around 160 km beneath the surface.