The Laboratory Manual Harmonisation Committee, universally known as the LMHC, is a working group of seven major coloured-stone laboratories that meets to align the descriptive language, treatment terminology and origin reporting conventions used on their reports. It was constituted in 2001 to address an obvious problem in the trade: the same stone could leave different laboratories with materially different language about heating, clarity enhancement and country of origin, creating confusion for dealers and consumers.

Member laboratories

The current members are AGL (American Gemological Laboratories, New York), CISGEM (Italy), GIA (Gemological Institute of America), GIT (Gem and Jewelry Institute of Thailand), Gübelin Gem Lab (Switzerland), GRS (GemResearch Swisslab, Switzerland), and SSEF (Swiss Gemmological Institute). Membership is closed; admission requires unanimous agreement from existing members and demonstration of equivalent technical capability and ethical standards.

Information Sheets

The committee publishes Information Sheets that document agreed wording on specific issues. As of writing, twenty-two Information Sheets are public. They cover, among other matters, the description of heated and unheated corundum, the disclosure of beryllium-diffused sapphire, the wording for clarity-enhanced emerald and the categories of heat treatment in tanzanite and aquamarine. Where members find an LMHC consensus, all member labs use the same language on their reports for that specific issue. Where consensus has not been reached, members continue to use their own house wording.

For the trade, the practical effect is that an LMHC member lab's description of "no indications of heating" or "H(a)" treatment carries the same meaning at any of the seven labs, which simplifies cross-laboratory comparison and reduces the risk of arbitrage between report wordings. LMHC documents are publicly available at the committee's website and are referenced in laboratory training materials worldwide.