Heated, Treated or Natural? Gemstone Treatments You Should Know About
Sep 2, 2025
The honest starting point: the great majority of coloured gemstones sold anywhere in the world have been treated in some way. That is not fraud. Fraud is treating a stone and not disclosing it.
Heat treatment
What it is: the stone is heated to high temperature to improve colour and clarity - often dissolving internal silk in corundum.
Common in: sapphire, ruby, aquamarine, tanzanite, citrine, amethyst.
Acceptability: universally accepted in the trade, and permanent. It has been done for centuries.
Price impact: substantial. An unheated fine sapphire can be worth several times a heated one of the same appearance.
Astrological view: most practitioners accept heated stones. Some traditions prefer unheated where budget allows.
Diffusion treatment
What it is: colouring elements - beryllium, titanium - are diffused into the surface at high temperature. The colour is a shallow layer, not throughout the stone.
Common in: sapphire, particularly bright orange-pink "padparadscha" material.
Acceptability: low. The colour can be lost if the stone is repolished.
Price impact: severe. Worth a small fraction of an equivalent untreated stone.
Astrological view: generally not accepted.
Glass filling
What it is: heavily fractured material is flooded with molten lead glass, which fills cracks and dramatically improves apparent clarity.
Common in: ruby. Also seen in sapphire.
Acceptability: very low. The stone is structurally compromised - it can be damaged by a jeweller's torch during ordinary repair, and by household acids.
Price impact: extreme. Glass-filled ruby is a low-value product sold at prices that suggest otherwise.
Astrological view: not accepted.
If a large, clean, vividly red "natural ruby" is affordable, it is glass filled. This is the most common gemstone deception in the Indian market.
Oiling and resin filling
What it is: cedar oil or a polymer resin is drawn into surface-reaching fractures to reduce their visibility.
Common in: emerald - the overwhelming majority.
Acceptability: minor oiling with natural cedar oil is standard and accepted throughout the trade. Significant oiling and polymer filling are disclosed separately and reduce value considerably.
What to ask: the grade - none, minor, moderate or significant. Treatment can degrade with heat and time, which is why oiled emeralds must never see an ultrasonic cleaner.
Dyeing
What it is: colour introduced into a porous material.
Common in: coral, turquoise, howlite, jade, agate, low-grade quartz.
Acceptability: low, and often undisclosed at the lower end of the market.
How to spot it: colour concentrated in cracks and around drill holes, unnaturally uniform tone, and colour that transfers to acetone on a cotton bud.
Astrological view: not accepted - particularly relevant for Red Coral, where dyed material is widespread. See our Red Coral guide.
Irradiation
What it is: exposure to radiation to alter colour, usually followed by heating.
Common in: blue topaz, some diamonds, smoky quartz, some pearls.
Acceptability: accepted where disclosed. Reputable material is tested for residual radioactivity and is safe.
Synthetic and lab-grown
What it is: grown in a laboratory. Chemically, physically and optically identical to natural material - not an imitation.
Common in: corundum (ruby and sapphire), emerald, spinel, quartz, diamond.
Acceptability: entirely acceptable as jewellery, and honestly priced when sold as what it is.
Astrological view: not accepted. The tradition holds that the stone must have formed in the earth. Because synthetics are visually indistinguishable, a laboratory report is the only protection.
What to ask before buying
- Is this stone natural or lab-grown?
- What treatments has it undergone?
- Is the treatment stated on the certificate?
- For emerald: what is the oiling grade?
- For ruby: has it been glass filled?
- For coral and turquoise: has it been dyed or stabilised?
- Which laboratory issued the report, and can I verify the number independently?
See how to read a gemstone certificate, and browse certified gemstones.
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