The H(c) code in Lotus Gemology's framework signals a stone that has been heat-treated and that contains significant residues in surface-reaching fissures or cavities. The treatment is principally thermal, but the residue load is large enough to be material to the stone's appearance and value, sitting at the upper end of acceptable conventional heat treatment before the disclosure crosses into composite or fracture-filled territory.

Where H(c) sits in the spectrum

H(c) is the third step in the heat-only ladder: H is heat with no detectable residue, H(a) heat without infilling, H(b) heat with minor residue, H(c) heat with significant residue. Beyond H(c), the laboratory description shifts to terms such as 'lead-glass filled' or 'composite ruby' to make clear that the filling material is doing structural work rather than acting as a flux byproduct.

Diagnostic features

Under microscopy, H(c) stones typically show flux residue that pools in fissures, sometimes with gas bubbles trapped within the residue and sometimes with iridescent flash effects along the residue boundary. Where the residue is borax-derived, FTIR spectroscopy identifies the diagnostic hydroxyl bands; where the residue is more glassy, Raman or LA-ICP-MS analysis can confirm composition.

Commercial treatment

An H(c) stone is a legitimately marketable heated stone, but it sits at a notable discount to H(a) and H(b) material at every quality tier. The discount widens with stone size and with target market: top-end retail and auction buyers are increasingly residue-averse, while middle-market goods continue to absorb H(c) material at appropriate prices. Disclosure must be explicit at every link of the supply chain.