Gemstone Treatments: What Every Buyer Should Know

Here is something many buyers are never told plainly: most coloured gemstones on the market today have been treated in some way. Heating, oiling and other processes are part of how the gem trade has worked for centuries, and many are completely accepted.

A treated stone is not a fake. A treated stone sold without telling you is the problem. International trade standards, such as those set by CIBJO, the World Jewellery Confederation, expect sellers to disclose treatments. This guide explains the common ones, why they matter, and what to ask before you buy.

Why treatments matter

A treatment can affect three things that matter to you:

• Value: an untreated stone of the same appearance is usually worth more, sometimes much more.
• Care: some treated stones need gentler cleaning and handling.
• Setting and repairs: your jeweller needs to know, because heat or chemicals used at the bench can damage some treated stones.

The most common treatments

Heating

The most widespread treatment of all. Controlled heat can improve colour and clarity, and the result is permanent. It is routinely used on sapphire and ruby; almost all tanzanite on the market has been heated to bring out its violet-blue; much aquamarine is heated to remove greenish tones; and a lot of citrine is heated amethyst. Heating is broadly accepted in the trade, though unheated stones of top quality command a premium.

Oiling and resin filling

Emeralds almost always contain small fractures that reach the surface. Filling them with oil or resin makes them less visible. This has been done for centuries and is expected in emerald. Filled stones should not go in an ultrasonic cleaner or near strong solvents, and oil can slowly dry out over many years.

Glass filling

Some lower-grade rubies, and some sapphires, have fractures filled with glass that contains lead. This can transform very included material into something that looks attractive, at a much lower price. Stones with large amounts of glass filling are properly described as composite or glass-filled, not simply as ruby. They are easily damaged by heat and by acids, including jeweller’s pickle and even lemon juice, so your jeweller must know before setting or repairing them.

Irradiation

Used to create or deepen colour. Almost all blue topaz is irradiated, as are some pink tourmalines and some coloured diamonds. Stones sold through proper channels are tested to be safe, and the colour is generally stable.

Diffusion

Elements are diffused into a stone at high temperature to change its colour, most notably beryllium diffusion in sapphire. Diffusion should always be disclosed, as it significantly affects value.

Dyeing

Common in beads and porous stones: agate, howlite, jade and quartz are often dyed, and some lower-grade rubies and emeralds are dyed to strengthen their colour. Dyed colour can fade or rub off over time.

Stabilisation and coating

Soft, porous turquoise is often stabilised with resin to make it durable enough to wear. Thin surface coatings create effects such as the rainbow of “mystic” topaz and some colours in raw diamond beads; coatings can wear away with use.

A dealer’s honest rule of thumb

Price tells you a lot. A large, richly coloured ruby or emerald at a modest price is almost certainly treated or of commercial grade. That is perfectly fine: these stones make beautiful, affordable jewellery. What matters is that you know what you are buying, so you can price your finished piece fairly and care for it properly.

Questions to ask before you buy

• Is this stone treated, and if so, how?
• Is the treatment permanent, and does it need special care?
• Is there a laboratory report? For valuable stones, an independent report is worth having.
• Is there anything my jeweller should know before setting it?

How we handle it

We state a stone’s treatment status on each listing wherever it is known. If a listing does not mention treatment, it does not mean the stone is untreated. Simply contact us before buying and we will tell you everything we know about that stone. We would rather answer every question than have a single customer surprised later.

Browse our loose gemstones with confidence, and treat every honest question as part of buying well.