Heat treatment is controlled heating of a gem to change its color or clarity.
In more detail
It is used on ruby, sapphire and amethyst, among others. GIA considers heated gems durable and permanent in normal wear. Most citrine is heated amethyst. Heating with an added element, such as beryllium, is a separate treatment: diffusion.
On a grading report
Corundum: 'indications of heating (H)' or 'no indications of heating (NH)'.
Related terms
- Indications of heating: 'Indications of heating' (H) is the standard LMHC lab wording for a corundum that shows signs of heat treatment. The heating changed its color or transparency.
- No indications of heating: 'No indications of heating' (NH) is the standard lab wording for a corundum that shows no signs of heat treatment.
- Residues in healed fissures: Residues in healed fissures are remains of the heating process, often glassy. They stay in fissures that healed while a corundum was heated.
- Beryllium diffusion: Beryllium diffusion is heating corundum with an added element, such as beryllium. The element diffuses into the crystal and changes its color.
- Silk: Silk is a mass of fine, crossing needle inclusions, usually rutile, in ruby and sapphire.