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Some three hundred years later Pliny the Elder Roman author naturalist and philosopher also noted the effects that heat had on stones. Theophrastus went on to classify stones according to how they reacted to heat.
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The mineral known today as amethyst plus many others was observed to change colour when heated.
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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. He wrote "some can be burnt whilst others can be melted and then there are those which just break up into smaller pieces." The Ancient Greek philosopher Theophrastus wrote in depth on this subject in his treatise 'Theophrastus on Stones'. 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. He would have been aware from quite early on that when heated, some stones also changed colour. With the passage of time man continued to learn about the effects of fire on stone. The purpose was to improve the quality and efficiency of stone tools that were being produced. 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. Heating rocks minerals and gemstones for the purpose of enhancing or changing their colour can be traced back thousands of years.Īncient man was a master of fire. Heating Rocks Minerals and Gemstones Heat Treatments Date Back Thousands of Years