Gemstones are often treated to improve colour, clarity, or durability. Some treatments are common and widely accepted, while others are less so. Here’s a quick guide to the treatments you’re most likely to see when buying gemstones.
Bleaching
Lightens or removes unwanted colour using chemical solutions. Common in jadeite, pearls, and chalcedony. Often followed by dyeing or impregnation. Disclosure is essential; acceptability varies by gem.
Colour Coating
Applies a thin layer of colouring to change or enhance the gem. Can scratch or wear off. Acceptable when used for unique, non-natural colours, e.g. Mystic Topaz.
Composite Gems (Doublets & Triplets)
Made by joining two or more layers to improve colour, clarity, or durability. Common with opals and fractured rubies. Less valuable than single stones and must be disclosed.
Diffusion
Adds or enhances colour near a gem’s surface using heat and chemicals. Colour is more durable than simple colour coating but it’s still shallow and can be lost if the gem is recut, so disclosure is important.
Diffusion with Beryllium
Beryllium penetrates deeper than standard diffusion, creating more durable colour, especially in sapphires. It’s controversial if undisclosed.
Dyeing
Adds colour to porous or pale stones, like agate, howlite, jade, or lapis lazuli. Colour can fade or leach out; dyed stones generally have a lower value and the treatment requires disclosure.
Heat
Heating gemstones is usually done to improve or change the gems colour, or enhance it’s clarity. Examples include turning amethyst into citrine or reducing silk inclusions in sapphires. It’s very common and widely accepted.
HPHT (High Pressure, High Temperature)
Used mainly on diamonds, HPHT improves colour or creates fancy hues by simulating natural formation conditions. Generally accepted in the trade.
Irradiation
Uses high-energy radiation to change colour, such as producing blue topaz from colourless topaz. Safe to wear and leaves no viable indications of the treatment so advanced lab testing would be needed to detect it.
Laser Drilling
Removes dark inclusions in lower-clarity diamonds using a tiny laser tunnel and bleaching. Improves clarity; holes may be visible under magnification. Generally considered acceptable if disclosed.
Fracture Filling
Fills surface-reaching fractures with substances like glass, resin, or oil to improve appearance. Common in rubies and emeralds but reduces durability; disclosure is essential.
– Oiling
Fills fractures with oil (often cedarwood) to improve clarity. Traditional and widely accepted in emeralds, though heavier or synthetic treatments should be disclosed.
– Glass-Filled Rubies
Lead-glass is injected into heavily fractured rubies to improve transparency and colour. These stones are less durable, considered composite, and far less valuable than untreated or heat-treated rubies.
Impregnation / Stabilisation
Porous stones like turquoise or opal are filled with wax, resin, or plastic to strengthen them and improve polish. Common and generally accepted when disclosed.
Sugar / Smoke
Enhances opals: sugar treatment darkens the background via sugar + acid bathing; smoke treatment uses carbon or smoke. Commonly accepted for Australian matrix opal but considered less acceptable for standard opals.