The Use of Heat on Rocks and Minerals
Heat treatments have been used to enhance or change the colour of rocks and minerals for thousands of years. It's well known that ancient man was a master of fire and 72,000 years but possibly as long as 164,000 years ago in South Africa fire was being used to heat stone in order to change its properties. The purpose for doing this was to improve the quality and efficiency of stone tools that were being produced. The application of heat to change the chemical make-up of a crystal rock or mineral is usually referred to as a heat treatment.
With the passing 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, certain types changed 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 had on rocks and minerals. He documented 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 when exposed to moisture saying that depending on the texture, some react differently when wet and dry. He noted that volcanic glass (obsidian) became porous when burnt and that colour and density also changed. The colour of amethyst and many other rocks and minerals was also noted to be affected by heat. Theophrastus went on to classify materials according to how they reacted when exposed to heat.
Pliny the Elder Roman author, naturalist and philosopher also wrote about the effects that heat 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. His statements indicate that he was aware that with the application of heat certain rocks and minerals changed or lost colour.
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. Although blue topaz does occur naturally it's extremely rare so the vast majority is produced by heat colourless stones. Depending on the type of treatment that's applied, it produces the London, Swiss or Sky Blue varieties.
Amethyst is often heated to change the colour to yellow so that it mimics the gemstone citrine which in its natural form is quite rare. Banded amethyst also known as chevron amethyst may be dyed or heated to produce prasiolite. The greener varieties of aquamarine are usually heated to change their colour into a lighter shade of blue and the mineral tourmaline which can often be quite dark becomes lighter after having been heated. Many of the world's finest rubies and sapphires are also heated to improve colour, clarity or both and those which have not been heated are considered to be extremely unusual. Exceptionally rare tanzanite is another mineral that's often heated to change and enhance the depth of colour and also to remove any undesirable tints of yellow or brown.
The main reason for using heat treatments to enhance or change the colour of rocks and minerals is to maximise beauty which subsequently makes it more desirable hence the value increases.