“No indication of heating” is the laboratory phrase used by SSEF, Lotus Gemology, and other LMHC-aligned coloured-stone laboratories to report that examination of a corundum stone (ruby or sapphire) detected no evidence of heat treatment. The phrase is precise and technically careful in a way that the casual trade phrase “unheated” is not: it reports what the laboratory observed rather than making an unconditional claim about the stone's treatment history. The distinction matters in the high-end coloured-stone market, where laboratory reports drive substantial price differentials and the language of those reports carries legal and commercial weight.

Why the phrasing is careful

A laboratory cannot directly observe the absence of treatment; it can only observe the absence of detectable indicators. The phrase “no indication of heating” acknowledges this epistemic limit explicitly. In practice, the indicators that laboratories look for are well established: partially dissolved or recrystallised silk inclusions, healed fractures with characteristic textures, surface diffusion of trace elements, infrared spectroscopic signatures of thermal modification of inclusions, and other features that develop in corundum exposed to high temperature. When these indicators are absent and the inclusion suite is consistent with a stone in its natural mineralogical state, the laboratory issues the “no indication of heating” conclusion.

The careful phrasing is increasingly important as treatment technology evolves. Low-temperature heating that produces minimal change to inclusions is harder to detect than the aggressive high-temperature treatments common in earlier decades. The LMHC laboratories have therefore standardised on language that reports observed evidence rather than asserting unqualified treatment status, allowing for the possibility that very gentle treatment might escape detection while still committing the laboratory to its observed result.

The LMHC framework

The Laboratory Manual Harmonisation Committee (LMHC) is an international body that brings together major gemmological laboratories — SSEF, Gübelin, GIA, AGL, GIT (Bangkok), CGL (Tokyo), and others — to coordinate terminology, methodology, and reporting practice. The committee has produced harmonised guidance on treatment terminology, origin terminology, and the form of standard laboratory reports for corundum, beryl, and other major coloured-stone species. The phrase “no indication of heating” is part of the LMHC's harmonised lexicon for corundum reporting and has been adopted across the LMHC laboratories.

Synonymous and related terms

Several terms are used in the trade to describe the same essential condition:

• Unheated. The colloquial trade term, simpler and more marketable but less technically precise. Used widely in marketing copy, dealer sheets, and informal communication.
• No thermal enhancement (NTE). A formal alternative phrasing used by some laboratories.
• No heat (NH). A common abbreviation in trade communication and sometimes in laboratory shorthand.
• Natural colour. A broader term that encompasses no-heat status but also includes other treatment-free conditions.

For practical purposes the distinctions between these terms are minor. A laboratory report stating “no indication of heating” supports a dealer's marketing of the stone as unheated, and the trade understands the equivalence.

The opposite finding

When the laboratory does detect indicators of heat treatment, the report uses parallel language: “indications of heating present” or “heat treatment indicated.” For aggressive treatments, more specific language identifies the type of treatment: "heat with residues" indicates a high-temperature flux-borne treatment; "surface diffusion" indicates a coating-and-firing process; "beryllium diffusion" indicates the controversial 2001-era treatment that adds beryllium to the lattice surface to produce padparadscha-like colour in formerly pinkish material. Each of these treatments has different price implications, with progressively more aggressive treatments commanding progressively lower prices.

Stones excluded from no-heat conclusions

For stones where indicator features are sparse or where examination cannot reach a confident conclusion, laboratories may issue a more cautious report. Common reasons include: very small stones with limited inclusions for examination, very included stones where features are obscured, stones cut in styles that limit microscopic visibility, and stones with treatment indicators that are ambiguous between natural and treatment-related. In such cases the report may say “heat treatment cannot be excluded” or omit a heating conclusion altogether, with the stone falling into a different (and lower) market price tier as a result.

In the trade

For working dealers, the practical importance of the “no indication of heating” phrasing is that it is the language that supports the unheated marketing claim with laboratory backing. Dealers and clients should look for this exact phrasing (or its accepted equivalents) on the laboratory report when buying or selling unheated corundum. Reports that use ambiguous language, that omit the heating conclusion, or that are issued by laboratories outside the LMHC core group should be evaluated on their own merits and may not support the same market premium.

Further reading

• SSEF — corundum reports and treatment terminology
• Lotus Gemology — heat treatment of corundum
• Gübelin Gem Lab — heat treatment terminology
• LMHC — harmonised laboratory terminology