Common Gemstone Treatments: What They Are and Why They're Used
Gemstone treatments are everywhere in the gem trade.
Some are widely accepted and even expected, like heat treating sapphires, while others are more controversial, especially if they're not properly disclosed. As a jeweller or gemstone buyer, understanding these treatments helps you make informed decisions and spot signs of enhancement when testing gemstones yourself.
Let’s take a look at the most common treatments, why they’re used, and how accepted they tend to be.
Heat Treatment
Heat treatment is when a gemstone is exposed to high temperatures to improve its appearance. This can enhance or change the gem’s colour (like turning amethyst into citrine). Or improve clarity by reducing inclusions, sapphires often benefit from this as heating can dissolve silk inclusions.
It’s one of the most common treatments and in most cases is accepted across the trade.
HPHT (High Pressure, High Temperature)
HPHT is mainly used on diamonds to either improve colour or create fancy colours like pinks and yellows. It mimics the natural conditions under which diamonds form and can remove brown tints or introduce desirable hues.
HPHT is a relatively common diamond treatment, and is generally considered an acceptable treatment.
Diffusion Treatment
Diffusion treatment involves heating a gemstone with certain chemicals to add or intensify colour near the surface. The most common example is blue diffusion-treated sapphire, where the outer layer is coloured while the centre remains pale or colourless.
This treatment is less accepted, especially when undisclosed, because the colour doesn’t run deep so it can be lost if the gem is recut or scratched.
Surface Diffusion with Beryllium
Beryllium diffusion is a more aggressive form of diffusion treatment that can create deeper colour changes, especially in sapphires. Unlike standard diffusion, beryllium atoms penetrate deeper into the gem, making the colour more durable.
It’s harder to detect and generally considered controversial in the trade, particularly when sellers don’t disclose it.
Irradiation
Irradiation changes a gem’s colour using high-energy radiation. It’s often used to create blue topaz (which is originally colourless) and sometimes with diamonds to produce green, blue, or black tones.
Irradiated stones are generally safe for wear, but the treatment can be hard to detect without lab testing.
Laser Drilling
Laser drilling removes dark inclusions from diamonds by creating a tiny tunnel with a laser and bleaching the inclusion. It improves the stone’s clarity, but the drill holes can be seen under magnification.
Laser drilling is more often seen in lower-clarity diamonds and is generally considered acceptable, provided it has been properly disclosed.
Fracture Filling
Fracture filling is when a substance (like glass, resin, or oil) is used to fill surface-reaching fractures and improve the stone’s appearance. This is often seen in rubies and emeralds, where the fill makes the stone appear cleaner and less included. Fracture filled stones are not as durable as non-filled stones.
While fracture filling is a common treatment, it's not commonly considered acceptable by many gem buyers.
Side Note on Glass-Filled Rubies
Glass-filling is a treatment used on heavily fractured rubies, where lead-glass is injected into the stone to improve clarity. These rubies can appear far more transparent and vibrant after treatment. However, the process leaves them much less durable.
Because of the extent of alteration, these stones were reclassified in the mid-2000s as composite gemstones rather than true rubies. They’re far less valuable than untreated or even heat-treated rubies and should always be clearly disclosed.
Oil Treatment (Mostly Emeralds)
Oiling is a traditional treatment for emeralds where clear oil (often cedarwood oil) is used to fill surface-reaching fractures. It improves clarity and appearance but doesn’t make the stone stronger.
Oiling is usually accepted in emeralds, but heavier oiling or use of synthetic resins should be disclosed, as it affects both value and care requirements.
Impregnation (sometimes called Stabilisation)
Impregnation involves filling a porous stone, like turquoise or opal, with colourless wax, resin, or plastic. This helps stabilise/strengthen the gem, making it more durable and improving polish.
It’s most commonly seen in turquoise and is generally accepted in the trade, provided it’s disclosed.
Bleaching
Bleaching is a treatment used to lighten or remove unwanted colour, commonly applied to gemstones such as jadeite, pearls, and some forms of chalcedony. The process typically involves soaking the stone in a chemical solution to reduce tints, stains, or uneven colouring.
In many cases, particularly with jadeite, bleaching is followed by dyeing or impregnation to improve appearance and stability. Because these treatments can affect a gem’s long-term durability and value, full disclosure is essential.
The acceptability of bleaching varies significantly by gemstone type. It is widely accepted in pearls, where whitening is considered standard practice, but may be viewed more cautiously in other stones.
Dyeing
Dyeing involves adding colour to a porous or light-coloured stone using dye. It’s commonly used on agate, howlite (to mimic turquoise), and sometimes lower-quality jade or lapis lazuli.
Because the colour isn’t natural and can fade over time or leach out, dyed stones are often considered lower value and are not as widely accepted by buyers as other colour altering treatments.
Colour Coating
Coating is when a gemstone is covered with a thin layer of material to change or enhance its colour. Mystic topaz is a great example of this practice.
This treatment can create dramatic colour shifts, but the coating is shallow and can scratch or wear off. It’s not always easy to detect, but the surface often shows a mottled or iridescent look under magnification.
Acceptance varies, but when it's used to give a stone unique colouring that's not seen in nature, such as with mystic topaz, colour coating is generally considered acceptable.
Sugar or Smoke Treatments
Though less commonly encountered than other treatments, sugar and smoke treatments are known techniques used to enhance opals.
Sugar treatment involves soaking the opal in a sugar solution, followed by exposure to acid, which darkens the background and improves the contrast of the play-of-colour.
Smoke treatment exposes the opal to smoke or carbon particles, which similarly darken the body tone to create a more vibrant appearance.
These treatments are not widely accepted when applied to standard opals, especially if undisclosed. However, in the case of matrix opal from Australia, particularly Andamooka matrix, they are common and generally accepted. In this context, the treatments are seen as a traditional method of significantly enhancing the stone’s visual appeal.
Final Thoughts
Treatments don’t make a gemstone “fake”, they just make it enhanced. But as with anything in gemology, disclosure matters.
If you’re testing gemstones at home, keep in mind that treatments can affect readings and appearance. Learn what signs to look for under magnification, and if something looks too perfect (or too weird), it might be time to investigate further.