Gemstone Treatments Explained

A treatment is any human intervention after a stone leaves the ground, apart from cutting and polishing, and the common ones are well documented, widely practiced, and disclosed in different ways.

Heat

Heating is the oldest and most accepted treatment. Corundum is heated to dissolve the fine rutile needles that make a sapphire look milky, and to deepen or shift color. Tanzanite is heated to turn brownish rough into blue-violet. Amethyst heated past a certain point becomes citrine. Aquamarine is heated to remove a green cast.

Heating is usually stable and permanent, which is why it is generally accepted in the trade. It does not make a stone fake. It does mean the color you see may not be the color the ground produced, and it does mean a treated stone and an untreated one of the same species are not the same market.

Irradiation

Irradiation uses controlled exposure to change color by altering the crystal's color centers. Blue topaz is the best-known example, and the treatment can produce blues that do not occur naturally in the material. Some pink tourmaline and some colored diamonds are also treated this way, and irradiation is often followed by heating.

Irradiated color is generally stable, but a few irradiated materials can fade with prolonged exposure to strong light or heat. Ask which treatment was used on any intensely colored stone at a modest price.

Oiling and fracture filling

Emerald commonly leaves the mine with surface-reaching fractures, and filling them with a colorless oil or resin improves apparent clarity. That practice goes back a long way and is expected in the material, but it is not permanent. The filler can be removed by solvents, heat, or rough cleaning, and a stone sold as untreated should not have it.

The practical consequence is a care rule: an oiled emerald does not belong in an ultrasonic cleaner, in a steam cleaner, or near a jeweler's torch.

Fracture filling goes further. Rubies with heavy fracturing are sometimes filled with lead glass, which reduces the visual impact of the fractures and can add weight. The result is still corundum, but the filled areas behave differently from the host stone: they are softer, they can be damaged by acids and by heat, and a repair or resizing that involves a torch can destroy the stone.

Filled stones can look like fine material in a photograph. They are usually priced far below unfilled material of the same species, and disclosure is essential because the buyer's care requirements are different.

Diffusion and coatings

Diffusion drives color into a stone from the outside using high heat and a coloring element. In corundum, it produces a colored layer near the surface. The color can be deep or shallow depending on the method, and repolishing a diffused stone can remove the color it was treated to gain.

Coatings are applied to the surface rather than diffused into it. Coated topaz in vivid pinks and greens is common, and dyes are used on agate, low-grade jade, some turquoise, and some lapis lazuli. Dyes can fade, bleed, or rub off against skin.

How disclosure is supposed to work

In the United States, the Federal Trade Commission's guidance for the jewelry trade requires that treatments be disclosed clearly, and specifically requires disclosure where a treatment is not permanent or where it significantly affects value or requires special care. Long-accepted processes are widely understood, and sellers are still expected to state them.

The word "treated" on its own is not a disclosure, because it does not tell you what was done. Ask for the treatment by name.

Disclosure matters for three reasons: what the stone is worth, how you can clean and repair it, and what happens if you resell or insure it. A stone whose treatment was never stated is difficult to value later, and an insurer may decline a claim for damage the seller never told you was likely.

If a listing does not name the treatment, ask directly and in writing, because the answer changes both the price and the way you handle the stone.