Heat Treatments on Rocks Minerals and Gemstones

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 alter its properties. The purpose for doing this was to improve the quality and efficiency of stone tools that were being produced.

With the passage of time early humans continued to learn about the effects of fire on stone.  They would have been 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 depth about 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 its colour and density also changed.

He also noted that amethyst and many other stones changed colour when heated.

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



Some three hundred years later Pliny the Elder Roman author, naturalist and philosopher also noted the effects that heat had on stones.  He noted "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 that by heating certain rocks and minerals they changed or lost colour.

Today different heat treatments are used to make subtle improvements to the colour of rocks, minerals and gemstones or to change their colour completely.

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

Topaz can be heated in an industrial oven or by exposure to radiation.  Scientists believe gemstones that have been irradiated pose no risk to health.

Before blue topaz that has been heated can be handled it must be stored away securely until any measurable levels of radiation have decayed.

The mineral tourmaline which can be quite dark is often heated to lighten the colour.  It too can be irradiated or heated through more conventional methods. 

Amethyst is often heated to produce citrine.  Greener shades of aquamarine can be heated to turn the stone blue.

Most of world's finest rubies and sapphires are heated to improve colour, clarity or both.  It's extremely unusual for these stones not to be heated.

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