Treated vs Untreated Gemstones: What Every Treatment Means for Colour, Care and Price

What does 'treated' mean for a gemstone?

A treated gemstone is a natural stone that has been altered after mining to improve its colour, clarity or durability. The crystal still formed in the earth; the treatment changes how it looks or behaves. An untreated stone, often sold as 'unheated' or 'no oil', has only been cut and polished. Treatment is one of the first things to understand when you browse coloured gemstones and birthstones, because it affects the price, the care and the wording on any report.

Treated is not the same as man-made. A lab-created (synthetic) gem is grown in a laboratory with the same chemistry and crystal structure as the natural mineral. An imitation is a different material that only looks similar, such as red glass sold as ruby. Our guide to lab-created and imitation gemstones covers those separately. A heated sapphire is still a natural sapphire. A lab-created sapphire is not natural, whether or not it has also been heated.

The trade sometimes splits processes into 'enhancements' (traditional, stable ones such as heating) and 'treatments' (more invasive ones such as glass filling). The labels vary between countries and laboratories, so the safest approach is to ignore the label and ask exactly what was done to the stone. Every technical term used on this page is also defined in our plain-English jewellery glossary.

What are the most common gemstone treatments?

Here is a practical gemstone treatments list, from the gentlest and most permanent to the ones that need the most care.

Heating is the oldest and most accepted treatment. A heat treated sapphire is typically held at roughly 1,200–1,800 °C for hours or days, which can deepen a pale blue, lighten an inky one, and dissolve silk (fine needle-like rutile inclusions that make a stone look milky). Because silk is also what creates a six-rayed star, stones meant for star sapphires are usually left unheated; our page on how silk creates stars and cat's-eyes explains why. Tanzanite is heated at much lower temperatures, around 500–600 °C, to turn its brownish tint into blue-violet.

Diamonds have their own set of processes, such as high-pressure, high-temperature (HPHT) annealing, irradiation and fracture filling. They are covered in our guide to HPHT and irradiated colour diamonds. For emeralds, filling surface-reaching fractures with cedarwood oil or a resin is so routine that a 'no oil' emerald is a genuine rarity; our emerald gemstone guide goes into the grades.

Which gemstones are usually treated, and which are not?

Some gems are almost always treated, while a few are usually sold exactly as they came from the ground. This table shows the typical pattern in the market.

Gems that are rarely treated, such as spinel, garnet and many tourmalines, are one reason collectors are drawn to them. Our guide to rare collector gemstones such as spinel and demantoid garnet looks at those in more depth. Treatment is never the whole story: an untreated stone with poor colour is not better than a beautifully coloured heated one.

What is the difference between treated vs untreated gemstones?

Both are natural. The differences are rarity, price, how the stone behaves over time and what the paperwork says. Are treated gemstones real? Yes: a heated ruby is a real ruby. The question is whether the treatment is stable and whether it was disclosed.

For an everyday ring, many people happily choose a heated sapphire: the colour is permanent, the stone is just as hard, and the budget goes further. Others want an untreated stone because it is untouched, and that is a personal value, not a technical one. If you are weighing up colour for a proposal, our guide to choosing a coloured gemstone engagement ring walks through the options. Neither choice should be thought of as an investment; gem prices move with fashion, supply and the market.

How can you tell if a gemstone is treated?

Often you cannot tell with the naked eye. Gemmologists look for clues under magnification first, then confirm with laboratory instruments. Common signs include:

• Heating: broken-up or 'dotted' silk, melted crystal inclusions and discoid fractures (small disc-shaped cracks with glassy halos around included crystals).
• Glass or resin filling: the flash effect, a blue, orange or pink flash inside a fracture as the stone tilts, plus trapped gas bubbles in the filler.
• Diffusion: colour concentrated along facet edges or near the surface, best seen with the stone immersed in liquid.
• Dye: colour pooled in cracks and pores rather than spread through the body.
• Coating: an iridescent or uneven surface film and scratches that expose a different colour beneath.

Laboratories then use instruments to confirm. FTIR (infrared) spectroscopy detects resins and some heating signatures, UV-Vis-NIR spectroscopy reads colour causes, Raman identifies fillers and inclusions, and LA-ICP-MS measures trace elements such as beryllium in diffused sapphire. Our scientific guide to identifying gemstones explains how each tool works.

A report that says 'no indications of heating' means the lab found no evidence of heat, not that it is impossible. Gentle, low-temperature heating can leave few traces, which is why reports from established labs carry more weight than a seller's verbal assurance.

Are Australian gemstones treated?

Australia produces two gems most buyers ask about: sapphire and opal. Their treatment patterns are quite different.

Australian sapphire comes mainly from the central Queensland gemfields around Anakie, Rubyvale and Sapphire, and from the New England region of New South Wales around Inverell and Glen Innes. These sapphires are rich in iron, so many blues are very dark, almost inky. Heating is commonly used to lighten and brighten them. Australian parti sapphires (stones showing blue, green and yellow zones) and many yellows and greens are often sold unheated, because their colour is what buyers want already.

Australian opal from Lightning Ridge, Coober Pedy and Andamooka is normally sold as solid, untreated opal. The main exceptions are doublets and triplets (thin slices of opal glued to a backing, with or without a clear quartz or glass cap), which are assembled rather than treated, and Andamooka matrix opal, which is often 'sugar-acid' treated to darken the background so the play of colour shows. Queensland boulder opal keeps its natural ironstone backing and is not a doublet.

Origin and treatment are separate questions on a report, but labs often answer them together because the same inclusions help with both. Our guide to how labs work out where a gemstone was mined explains the overlap.

Do treatments change gemstone durability and care?

Heat and irradiation generally leave a stone as tough as it was. Fillers, dyes and coatings are another matter: they add a softer material that reacts to heat, chemicals and ultrasonic vibration. This table pairs each gem's natural durability with its usual treatment, so you can judge it for a ring. Hardness (Mohs scale) is resistance to scratching; toughness is resistance to chipping and breaking.

For the full ranking see our gemstone hardness guide on the Mohs scale, and for how hardness, toughness and stability combine on the hand, read durability for rings you wear every day. A few care rules follow directly from treatment:

• No ultrasonic or steam for oiled emeralds, glass-filled rubies, dyed, coated or stabilised stones, or opal.
• Warm soapy water and a soft brush is safe for almost every treated stone.
• Keep acids away from glass-filled ruby; even lemon juice and some household cleaners can etch the lead glass.
• Tell your jeweller about the treatment before resizing, re-tipping claws or repolishing. Torch heat can damage fillers, and repolishing can remove coatings or a shallow diffused layer.

The setting matters as much as the cleaning. A bezel or well-placed claws protect a filled or brittle stone from knocks; our guide on designing a secure setting for a coloured gemstone covers the details. Stone-by-stone cleaning instructions are in our care guide for each gemstone.

How much do treated vs untreated gemstones cost?

Treatment is one of the biggest price factors in coloured stones, alongside colour, clarity, cut, size and origin. The ranges below are broad indications for about 1 carat of good colour and clarity, to show the size of the gap rather than to price a specific stone.

The untreated gemstone price gap widens sharply with quality and size: at the top of the market, a fine unheated sapphire or no-oil emerald can cost several times a comparable treated stone. At the bottom, glass-filled ruby is priced closer to a manufactured product than to a natural ruby, which is why it must always be disclosed. Lab-created stones sit lower again and are a different category altogether.

If your priority is a hard-wearing stone for daily wear at a sensible price, a heated sapphire or an untreated spinel often makes more sense than chasing an untreated ruby. Our list of gemstones that cope well with daily wear compares the options.

What should a seller tell you about gemstone treatments?

Under the Australian Consumer Law, a seller must not mislead you, and leaving out a treatment that would matter to a reasonable buyer can be misleading. International trade codes, such as the CIBJO guidelines followed by many jewellers and laboratories, also call for treatments to be disclosed at every sale. In practice, good gemstone treatment disclosure means:

• The treatment named on the invoice or certificate, not just mentioned in conversation.
• The degree where it applies, such as 'minor' or 'moderate' clarity enhancement for an emerald.
• Any special care the treatment needs.
• Whether the stone is natural, lab-created or assembled (a doublet or triplet).

Report wording is fairly consistent between major labs. 'No indications of heating' means no evidence of heat was found. 'Indications of heating' means it was heated. 'Indications of heating, with residues in fissures' means heat was used and some foreign material remains in fractures. Some labs describe heavily glass-filled ruby as a 'composite' or 'manufactured product' rather than simply ruby, which tells you how much of the stone is filler.

Treated vs untreated gemstones: how to choose, step by step

1. Decide what matters to you. Untouched rarity, the most colour for your budget, or the toughest stone for daily wear.
2. Ask two questions. Has the stone been heated? Has it been filled, dyed, coated or diffused?
3. Get it in writing. The treatment, and its degree, should be on the invoice or a certificate.
4. Ask for a lab report on higher-value stones. Especially for anything sold as unheated, no oil or untreated, where the premium depends on the claim.
5. Look for yourself. Under a 10× loupe, tilt the stone and watch for flashes inside fractures or colour pooled in cracks.
6. Match care and setting to the treatment. Use our guide to diamond and gemstone settings to choose protection that suits the stone.
7. Keep the paperwork. Insurers and future jewellers will need to know the treatment.