The Lotus Gemology heat letter is the umbrella term for the H, H(a), H(b) and H(c) codes that the laboratory uses to grade thermally treated corundum and spinel by residue content. The series is calibrated so that buyers and dealers can read, at a glance, both that a stone has been heated and how much foreign residue, if any, the heating left behind in surface-reaching fissures.

The series

• H: thermally enhanced, no detectable residues.
• H(a): heated, no residues observed in surface-reaching fissures or cavities.
• H(b): heated, minor residues present.
• H(c): heated, significant residues present.

Beyond H(c) the disclosure leaves the heat-letter system altogether, becoming 'lead-glass filled', 'composite ruby', or similar language indicating that infilling rather than thermal enhancement is the principal modification.

Why the system was introduced

Pre-existing trade disclosure on heat treatment was binary - heated or unheated - which obscured the very real difference between a clean heated stone and one with substantial flux residue. The H-letter series gives the trade a vocabulary for that difference. It also provides cover for honest dealers, who can show clients explicit residue language rather than fielding a vague 'heated' label that buyers increasingly distrust.

Adoption

The H-letter system is published on Lotus reports for ruby, sapphire, and spinel. Other major laboratories use their own conventions; GIA and Gübelin distinguish unheated from heated and disclose evidence of residues separately, while AGL uses its own enhancement code. For buyers comparing reports across laboratories, the trade convention is to read each lab's language on its own terms rather than to map one system onto another.

Practical use

For a working dealer, the heat letter is the first line of triage on a coloured stone. An H or H(a) call places the stone in clean heated territory; H(b) and H(c) place it progressively further down the residue spectrum. The vocabulary has spread across Bangkok, Hong Kong and the major Western dealers since its introduction, and it is now common to hear traders refer to a stone informally as 'an H(a)' or 'a clean H' even before its laboratory paper is on the table.