Heat Treatments in Relation to Stones


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Heat treatments have been used to enhance or change the colour of rocks and minerals for thousands of years and it's long been known that ancient man was a master of fire. 72,000 years but possibly as long as 164,000 years ago in South Africa fire was being used in a controlled environment to heat stone in order to change its properties. The purpose of doing this was to improve the quality and efficiency of stone tools that were being produced. The application of of heat with the intention of changing properties within a crystal, rock or mineral is correctly known as a heat treatment.

With the passing of time man continued to learn about the effects that fire had on rocks and minerals and he would have been well aware quite early on that when heated, some would change colour.

The Ancient Greek philosopher Theophrastus (c.371 - c.287 BC) successor to Aristotle wrote in great depth in his treatise "Theophrastus on Stones" about the effects that heat treatments had on stones. He wrote that 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 that moisture had on stones saying that depending on their texture some react differently when wet and dry.  He noted the mineraloid obsidian became porous when burnt and that its colour and density also changed and that amethyst, emerald and sapphire which is blue corundum lost colour when heated.

Pliny the Elder Roman author, naturalist and philosopher also wrote about the effect that heat treatments had on stones saying that one gemstone could be changed into another and the colour of crystal quartz could be changed into that of an emerald. Both of these statements indicate that what Pliny was describing was the effect that heat can have on certain rocks and minerals.

A variety of different heat treatments are used to both enhance and change the colour of stones and whilst the results in some may be relatively subtle, in others it can be quite dramatic. Topaz for example rarely occurs in blue so the vast majority of blue topaz is produced by heating the colourless variety.  Depending on the type of heat treatment that's used it can produce three different shades which are commonly known as London, Swiss or Sky Blue.  The mineral amethyst is often heated to mimic the yellow variety of quartz known as citrine and banded amethyst imitates prasiolite because both gemstones are relatively rare in their natural form. The greener varieties of aquamarine are heat treated in order to change their colour into a lighter shade of blue and the mineral tourmaline which can often be dark, becomes lighter with the application of heat. Some people may be surprised to learn that many of the world's finest rubies and sapphires are heat treated in order to improve colour or clarity and those which have not been heated are considered to be extremely unusual. Tanzanite a blue variety of the mineral zoisite is an extremely rare and highly sought after gemstone which comes exclusively from Tanzania.  Stones are regularly heated in order to enhance depth of colour and to remove any undesirable hues of brown and yellow.

Whilst some may frown upon the practice of heat treating rocks and minerals for the purpose of enhancing or changing their colour, the sole aim of this process is to maximise beauty which subsequently makes the stone more desirable hence its value also increases. What's important to note is that heat treatments are a relatively natural process which could be described as man copying nature but these treatments must not be confused with dyeing which is an artificial method of changing the colour of a stone using some form of colourant.


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