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Colour: a property that makes gems desirable but not necessarily identifiable.

Sapphires, rubies, diamonds ... words we associate with kings and queens, the rich and famous, glitz and glamour. Humans have been fascinated by these sparkly stones throughout all time. If the conditions are just right, Earth can produce large, transparent crystals that can be cut into gemstones. Actually, any mineral can be cut up to be used for jewellery, but the most desirable are minerals that are hard, resist wear and breakage, and have the richest of colours. Or, in the case of diamonds, the most colourless and clear are the most desired.

Gem cutters bring out the inner beauty of gems by creating small polished surfaces on the stone called facets, which play a trick with light. Facets allow light to enter the gem; once inside, the light bounces off its other angled surfaces. This causes the light to disperse, or split apart, into the colours of the rainbow before it leaves the gem, similar to what happens when you pass light through a prism. This is the sparkling colour you see when you look at a gemstone.

Sometimes gems have different names depending on their colour, though they may be varieties of just one mineral. For example, sapphires and rubies are actually the same mineral, corundum. The difference between the two is at the atomic level, something that can't be seen with the eye but that makes the colours different. Rubies are always red; sapphires can be any other colour than red, but blue is the most commonly known.

In the gem-cutting process, sometimes more than half the mineral can be lost, so gemstones are cut into different shapes to maximize their size. The bigger and more perfect the gemstone is, and the rarer it is on Earth, the more valuable it becomes.

COLOUR

Minerals have been used as pigments since the dawn of history, either in powdered form, as paint and cosmetics, or as sources of colour in ceramic glazes. Hematite and other iron oxides create reds and browns, while cinnabar and minium give us bright reds and malachite is a source of green.

Colour is one of the first things we observe about a mineral. Some minerals are found in only one colour, while others, such as fluorite, may have a wide range of colours and hues. In some cases, the colour is due to the chemical composition of the mineral itself, but it can be influenced by the presence of another mineral. As a result, colour is one of the least definitive properties used for identification of minerals.

Minerals with a characteristic colour are called idiochromatic minerals. Their colour is caused by one or more of the major elements. For example, rhodochrosite (MnCO3), regardless of where it is found in the world, is characteristically pink or red because of its manganese content. The characteristic green of malachite and blue of azurite are both due to copper, an element that can produce different colours in minerals with different crystal structures.

From the forthcoming book The Royal Ontario Museum Gems & Minerals by ROM mineralogist Kim Tait. Co-published by Firefly and Royal Ontario Museum Press. Available August 2011.

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RHODOCHROSITE

From Kuruman, Northern Cape, South Africa. Formula: MnCO3 Crystal system: hexagonal Space group: R3c Hardness: 3.5-4.0 Specific gravity: 3.70 Cleavage: {1011} perfect Fracture: uneven to subconchoidal Tenacity: brittle Notable locality in Canada: Mont-Saint-Hilaire, Quebec Name: from the Greek rhodon meaning "rose," and chrosis meaning "colouring."

ZIRCON

Teardrop-cut blue zircon. Source unknown. Zircon occurs in a range of colours but blue, golden brown, and white are the ones most often used for gemstones.

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ANGLESITE

From Tsumeb, Namibia. Anglesite is a rare lead mineral. The high lustre typically associated with lead minerals makes them beautiful gemstones.

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TOPAZ Source unknown. This rectangular mixed-cut imperial topaz gemstone is 159.1 carats.

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AXINITE-(FE) From Baja California Norte, Mexico. This 3.1 carat cut gemstone is from a find of 50 pounds (22.7 kg) of gem-quality axinite in Mexico.

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CUPRITE From Onganja, Namibia. Commonly called "ruby copper," cuprite is a secondary ore of copper that occasionally forms attractive ruby-like transparent gemstones.
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Title Annotation:In the Galleries
Author:Tait, Kim
Publication:ROM Magazine
Date:Jun 18, 2011
Words:692
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