The plain code H in Lotus Gemology's disclosure framework denotes a stone that has been heated but in which the laboratory has detected no residues whatsoever - no flux, no glass, no fingerprint of foreign material in surface-reaching fissures or cavities. The stone has been thermally enhanced and shows the standard inclusion features that disclose heating, but it is otherwise as clean as a heated stone can be.

Distinguishing H from H(a)

The framework distinguishes between a plain H call and the H(a) call. In Lotus's published explanation, H means heat-only with no evidence of any residue at all; H(a) is reserved for heated stones in which the laboratory has examined the fissures and found no residues during examination. The distinction is fine and, in practice, the two codes are often used near-interchangeably; for buyers and dealers, both denote the cleanest tier of heated material.

Diagnostic basis

An H designation rests on standard diagnostics for heat in corundum and spinel: altered silk, dissolved or recrystallised rutile, snow-balled remnants, discoid stress fractures, and modified colour banding. Spectroscopic features in UV-Vis-NIR and FTIR also contribute, particularly where heating has modified iron-titanium charge-transfer absorption or shifted hydrogen-related bands.

Trade significance

For ruby and sapphire, an H call places the stone in the standard heated tier. The premium for unheated material at top quality runs from a substantial multiple to many multiples of the heated price, which is why the boundary between unheated and H is one of the most consequential lines a laboratory draws. Stones that fall into the unheated category receive a different style of report altogether, with explicit 'no indications of heating' language.