Heating Rocks and Minerals
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 that volcanic glass also known as 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. He also classified minerals and gemstones according to how they reacted to the application of heat.
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. Blue topaz which rarely occurs naturally is produced by heating colourless topaz and depending on the type of treatment that's applied, it produces the London, Swiss or Sky Blue varieties. Amethyst the purple variety of quartz is often heated so that it turns yellow which mimics the gemstone known as citrine and banded amethyst is often dyed to imitate prasiolite because both gemstones are relatively rare in their natural form. The greener varieties of aquamarine are heated to change the colour into a lighter shade of blue and the mineral tourmaline which can often be dark becomes lighter after having been exposed to a heat treatment. Many of the world's finest rubies and sapphires undergo heat treatments to improve colour, clarity or both and those which have not been heated are considered to be extremely unusual. Tanzanite blue variety of the mineral zoisite is almost always heated to change and enhance the depth of colour and also to remove any undesirable tints of brown and yellow.
The primary reason for using heat treatments to enhance or change the colour of rocks and minerals is to maximise beauty which subsequently makes the stone more desirable hence its value will increase.