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Heating Rocks Minerals and Gemstones

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Heat Treatments Date Back Thousands of Years

Heating rocks minerals and gemstones for the purpose of enhancing or changing their colour can be traced back thousands of years.

Ancient man was a master of fire.  72,000 but possibly as long as 164,000 years ago in South Africa, fire was being used to heat stone to change its properties. The purpose was to improve the quality and efficiency of stone tools that were being produced.

With the passage of time man continued to learn about the effects of fire on stone.  He would have been well aware from quite early on that when heated, some stones also changed colour.

Today the application of heat in any form to change the chemical composition of a rock, mineral or gemstone is known as a heat treatment.

The ancient Greek philosopher Theophrastus [c.371 - c.287 BC] wrote in great depth on this subject in his treatise 'Theophrastus on Stones'. 

He wrote "some can be burnt whilst others can be melted and then there are those which just break up into smaller pieces."

He also described the effects of moisture on rocks and minerals saying "depending on texture some react differently when wet and dry."  He noted volcanic glass (obsidian) became porous when burnt and that colour and density also changed.

The mineral known today as amethyst plus many others was observed to change colour when heated.

Theophrastus went on to classify stones according to how they reacted to heat.

an extract from Theophrastus on Stones

About three hundred years later Pliny the Elder Roman author naturalist and philosopher also noted the effects that heat had on stones.  He wrote one gemstone could be changed into another and the colour of crystal quartz could be changed into that of an emerald.

These statements confirm he was aware with the application of heat certain rocks and minerals changed or lost colour.

Today different heat treatments are used to both enhance and change the colour of rocks, minerals and gemstones.  Whilst in some the results may be relatively subtle, in others it can be quite dramatic.

Although blue coloured topaz occurs naturally it's extremely rare.  The vast majority is therefore produced by heating colourless topaz.  Depending on the type of treatment three different shades of blue can be produced.

Amethyst is often heated to change the colour to yellow which mimics citrine.

When aquamarine with a greener shade of colour is heated, it changes to a light shade of blue.  When used for jewellery blue aquamarine is far more popular than green.

The mineral tourmaline which can be quite dark is often heated to lighten the colour.  

Most of world's finest rubies and sapphires are heated to improve colour, clarity or both.  Gemstones which have not been heated are considered to be extremely unusual.

Most tanzanite is heated to change or enhance the depth of colour.  It also removes undesirable tints of yellow or brown.

The primary reason for heating gemstones is to maximise beauty.  This makes them more desirable which in turn increases value. 

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