The AGTA Gemstone Enhancement Code is a standardised, letter-based disclosure system developed by the American Gem Trade Association (AGTA) to identify and communicate the treatments applied to coloured gemstones and cultured pearls at the point of sale. Adopted widely across the North American coloured-stone trade and referenced by major gemmological laboratories and trade publications, the system represents one of the most practical and enduring frameworks for treatment transparency in the modern gem market.

Purpose and Background

Gemstone treatments — from routine heating to more interventionist processes such as fracture filling or irradiation — can significantly affect a stone's appearance, durability, and value. Buyers, whether trade professionals or end consumers, have a legitimate interest in knowing what enhancements a stone has received. The AGTA enhancement code was developed to give sellers a concise, consistent vocabulary for making those disclosures, reducing ambiguity and supporting informed purchasing decisions. The code forms a core component of the AGTA's broader ethical sourcing and disclosure guidelines, which member firms are required to follow.

The Enhancement Codes

Each letter in the system corresponds to a specific enhancement process. The current codes, as published by the AGTA, are as follows:

• B — Bleaching: The use of heat, light, or chemical agents to lighten or remove a stone's colour. Commonly applied to jade, pearls, and coral.
• C — Coating: The application of a surface layer — lacquer, plastic, wax, or other substance — to alter colour or improve appearance. Seen on certain topaz, quartz, and drusy materials.
• D — Dyeing: The introduction of colouring matter into a porous or fractured stone to change or intensify colour. Frequently encountered in jade, lapis lazuli, turquoise, and some rubies.
• F — Filling: The use of glass, resin, or other materials to fill surface-reaching fractures or cavities, improving apparent clarity and, in some cases, colour. Lead-glass filling of rubies is a well-documented commercial example.
• H — Heating: The application of controlled heat to alter colour, clarity, or both. The most prevalent treatment in the coloured-stone trade, routinely applied to sapphire, ruby, aquamarine, tanzanite, and many other species.
• I — Impregnation: The infusion of a colourless substance — typically a polymer or resin — into a porous stone to improve stability and surface appearance. Turquoise is the most common subject.
• L — Laser Drilling: The use of a laser beam to create fine channels into a stone, typically to reach and bleach dark inclusions. Primarily associated with diamonds, though the code is retained within the broader system.
• N — No Treatment / Not Enhanced: The stone has received no artificial enhancement of any kind. This designation carries significant commercial weight, particularly for rubies, sapphires, and emeralds, where untreated material commands substantial premiums.
• O — Oiling / Resin Infilling: The filling of surface-reaching fractures with a colourless oil, resin, or wax — most classically cedar oil in emeralds — to improve apparent clarity. The jardin of an emerald is frequently treated in this manner, and the degree of oiling is considered material to value.
• R — Irradiation: Exposure to a radiation source (gamma rays, neutrons, or electrons) to alter colour, often followed by heating. Applied to blue topaz, some diamonds, and certain fancy-colour sapphires and tourmalines.
• S — Stabilisation: The combination of impregnation with a bonding agent to consolidate a fragile or porous material. Turquoise and some opals are stabilised to prevent deterioration.
• U — Undisclosed Enhancement: The seller is aware that a treatment has been applied but cannot identify its precise nature. Use of this code signals the need for laboratory examination before significant purchase decisions are made.
• W — Waxing / Polishing: The application of a surface wax or polish to improve lustre. Common in jade and some opaque materials; generally considered a minor, traditional finishing step.

Application in the Trade

AGTA member firms are required to disclose enhancements using these codes on invoices and sales documents. The system is particularly valuable at trade shows such as the Tucson Gem Shows, where large volumes of stones change hands rapidly and buyers rely on concise, standardised notation. Major gemmological laboratories — including the Gemological Institute of America (GIA) and Gübelin Gem Lab — issue their own treatment disclosures on laboratory reports, and while their language may differ in detail, the underlying categories align closely with the AGTA framework.

It is worth noting that the codes describe type of treatment, not degree. The extent of fracture filling in an emerald, or the depth of colour change achieved by heating, requires a laboratory report for meaningful quantification. The AGTA codes are therefore a starting point for disclosure, not a substitute for expert gemmological examination when significant value is at stake.

Further Reading

• AGTA — Gemstone Enhancement Codes (agta.org)
• GIA — Gem Treatments Overview (gia.edu)