- Nature of gemstone: This technique is applied on diamonds.
- Possible changes: Poor quality yellow or brown coloured diamonds changes to black.
- Conditions: Exposure of diamonds to high temperature ranging from 900°C - 1650°C in a vacuum, for a period ranging from few minutes to several hours.
- Cause: Due to increase in temperature, without the application of pressure, diamond starts turning into graphite, especially where there is some spaces in the stone like cleavage or feather.
- Identification: Concentration of black to gray graphite along the fractures and cleavages, sometimes form black lines along the edges of fractures or cleavages.