Within the American Gem Trade Association's system of enhancement disclosure, Code H designates heat treatment — the controlled application of elevated temperatures to a gemstone with the intention of permanently altering its colour, clarity, or optical phenomena. It is the single most prevalent enhancement code in the coloured-gemstone trade, applied to the majority of commercially available rubies, sapphires, tanzanites, aquamarines, and a broad range of other species. Understanding Code H is foundational to any serious engagement with gemstone valuation, laboratory reporting, and ethical disclosure.

The AGTA Enhancement Disclosure System

The AGTA introduced its system of standardised enhancement codes to bring transparency and consistency to a trade in which treatments had historically been inconsistently disclosed or silently assumed. The full schedule of codes — running from B (bleaching) through U (diffusion) — assigns a single letter to each category of enhancement, enabling dealers, retailers, and consumers to communicate treatment status without ambiguity. Code H sits at the centre of this framework, both alphabetically and commercially. The AGTA requires member dealers to disclose enhancements at the point of sale, and Code H is the designation most frequently appearing on invoices, laboratory certificates, and auction catalogue notes.

The code is not a judgement of quality or desirability; it is a factual statement about process. A fine heated Burmese sapphire of exceptional colour and clarity remains a fine sapphire — the code simply ensures that the buyer knows its history.

What Heat Treatment Does

The application of heat to gemstones exploits the thermodynamic sensitivity of chromophore elements and structural defects within a crystal lattice. The specific outcomes vary considerably by species:

• Corundum (ruby and sapphire): Heat dissolves silk — fine rutile needles — improving transparency and, in blue sapphires, intensifying or homogenising colour by redistributing iron and titanium charge-transfer pairs. In rubies, heat can reduce brownish or purplish secondary hues, moving colour closer to a pure red. Temperatures typically range from approximately 1,600°C to 1,850°C for corundum, often in controlled atmospheric conditions (oxidising or reducing) to achieve the desired result.
• Tanzanite (zoisite): Natural tanzanite is predominantly brownish to reddish-purple in its rough state. Heating to approximately 600°C in an oxidising environment eliminates the brown component, producing the saturated violet-blue for which the species is commercially known. The treatment is so universal that unheated tanzanite of gem quality is a rarity rather than a norm.
• Aquamarine (beryl): Greenish or yellow-green aquamarine is routinely heated to approximately 400–450°C to remove the yellow component, yielding a purer, more commercially desirable blue. The process exploits the differential stability of iron oxidation states within the beryl structure.
• Tourmaline: Certain brownish or dark tourmalines, particularly from Mozambique and Nigeria, are heated to lighten tone or shift hue toward more marketable pink or red ranges.
• Spinel, zircon, and others: Heat is applied selectively across many other species to modify colour or reduce undesirable secondary tones.

Permanence and Stability

A defining characteristic of Code H treatment — and a principal reason for its broad acceptance — is its permanence under normal conditions of wear, cleaning, and jewellery repair. Once a corundum crystal has been heated and its rutile silk dissolved or its colour centres stabilised, the change does not reverse at ambient temperatures or under the brief, localised heat of a jeweller's torch during routine setting work. This distinguishes heat treatment fundamentally from treatments such as fracture filling or surface coating, which are fragile, reversible, or subject to degradation. The GIA, Gübelin Gem Lab, and SSEF all acknowledge heat treatment as a stable, accepted enhancement in their grading and reporting frameworks.

Detection and Laboratory Reporting

Determining whether a gemstone has been heated — and, critically, whether it has not been heated — is among the most technically demanding tasks in applied gemmology. Laboratories employ a combination of methods:

• Microscopic examination: In corundum, the presence or absence of intact rutile silk, the morphology of any residual silk, the appearance of stress fractures around inclusions (caused by differential thermal expansion during heating), and the condition of crystal inclusions all provide evidence of thermal history.
• UV-Vis-NIR spectroscopy: Characteristic absorption features in corundum, particularly the 450 nm band associated with iron in sapphire, can shift or intensify with heating.
• Photoluminescence spectroscopy: Certain emission features in ruby and sapphire are sensitive to heat, and their presence or absence informs the laboratory's conclusion.
• Infrared spectroscopy: Used particularly in beryl and other species to detect structural changes associated with heating.

The GIA issues sapphire and ruby reports that explicitly state "No indications of heating" or "Indications of heating" (with a further qualifier noting whether residues consistent with flux or glass are present, which would invoke a separate, more serious disclosure). Gübelin and SSEF use comparable language. A report stating no indications of heating is a prerequisite for the highest market premiums in fine ruby and sapphire.

Market Implications and Premiums for Unheated Stones

Code H treatment is so standard in the corundum trade that the absence of heat — rather than its presence — is the market exception requiring documentation. A fine, unheated Burmese ruby or Kashmir sapphire of comparable colour and clarity to its heated counterpart will command a substantial premium, routinely 50–200% above the heated stone at the top of the market, and in exceptional cases considerably more. At major auction houses — Christie's, Sotheby's, Bonhams — the phrase "no indications of heating" in the accompanying laboratory report is a material factor in estimate-setting and realised prices.

The premium reflects scarcity as much as any intrinsic superiority. Nature produces far fewer rubies and sapphires whose colour is already optimal without thermal intervention. Collectors and connoisseurs value the unheated stone as a record of what the earth produced without human modification. This is a market and philosophical position rather than a gemmological one: a well-heated stone is not inferior in beauty, durability, or wearability.

For tanzanite, the calculus is different. Because virtually all gem-quality tanzanite is heated, the unheated designation carries less commercial weight, and the market does not consistently apply a premium for unheated material in the way it does for corundum.

Disclosure Obligations and Trade Ethics

The AGTA Code of Ethics requires member companies to disclose all known enhancements at the time of sale, using the standardised code system. Code H must therefore appear on any documentation accompanying a heated stone sold by an AGTA member. The Federal Trade Commission's Guides for the Jewelry, Precious Metals, and Pewter Industries similarly require disclosure of treatments that are not permanent or that require special care — though heat treatment, being permanent, occupies a somewhat different position in FTC guidance than, for example, fracture filling.

In practice, the trade has reached a broad consensus: Code H is disclosed, accepted, and priced accordingly. It is not a stigma. The ethical failure lies not in selling heated stones but in selling them without disclosure, or — more seriously — in representing a heated stone as unheated to obtain an unjustified premium.

Relationship to Other AGTA Codes

Code H should be distinguished from related but distinct designations. Where heating is accompanied by the introduction of a foreign substance — beryllium diffusion into corundum, for instance — the applicable code shifts to U (diffusion), a more invasive and less universally accepted treatment. Where a fracture-filled stone has also been heated, both codes apply. The AGTA system permits stacking of codes to reflect complex treatment histories, and reputable laboratories will note each applicable enhancement separately on their reports.

Further Reading

• AGTA Enhancement Codes — agta.org
• GIA on Gemstone Treatments — gia.edu
• Heat Treatment of Ruby and Sapphire — Gems & Gemology, GIA