'No indications of heating' (NH) is the standard lab wording for a corundum that shows no signs of heat treatment.
In more detail
It is the lab's finding from the stone's inclusions and spectra, not a guarantee. Some heating conditions leave certain inclusions unchanged. 'Unheated' is the trade word for the same claim.
On a grading report
Further information: 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.
- Heat treatment: Heat treatment is controlled heating of a gem to change its color or clarity.
- Silk: Silk is a mass of fine, crossing needle inclusions, usually rutile, in ruby and sapphire.
- Origin report: An origin report gives a lab's opinion on the country or region where a gem was mined.