Diamond treatments

Treatments fall into two groups. Clarity treatments make inclusions less visible, so a stone can look cleaner than its natural clarity grade suggests. Colour treatments change the body colour of the stone, either to improve a colour grade or to produce a fancy colour. The table summarises what each does, using GIA's descriptions.

HPHT stands for high pressure, high temperature. The same type of press can either alter a diamond's colour or grow a diamond. A treated natural diamond and a lab-grown diamond are different things: the first was mined and then altered, the second was never mined.

In the United States, the FTC's Jewelry Guides say a seller should disclose a gemstone treatment when it is not permanent, when it creates special care requirements, or when it has a significant effect on the stone's value. Diamonds are covered by that rule. In person, the disclosure can be made at the point of sale before the sale; for online listings it should be in the product description. These are US guides: they do not bind the public, although the FTC can still enforce the law they interpret.

In Canada, the Competition Act prohibits making a representation to the public, to promote a product or business, that is false or misleading in a material respect. Whether leaving out a treatment is misleading depends on the facts, so the practical move is to ask, and to get the answer in writing on the invoice or the report.

You usually cannot tell by eye. Fracture filling, laser drilling and colour treatments are identified by gemmologists with magnification and laboratory equipment. Three habits help:

• Ask the seller directly: "Has this diamond been treated in any way?"
• Ask for an independent grading report from a lab such as GIA and read it for treatment notes. GIA does not grade fracture-filled or coated diamonds, so these stones will not arrive with a GIA report.
• Match the report to the stone, using the laser inscription on the girdle if there is one.

One colour clue is worth knowing: Treated yellow diamonds often have intensely saturated colour that looks a bit cooler or greener than natural ones. It is a prompt to ask for the report, not proof.

Sometimes, if the treatment is disclosed and the price reflects it. A laser-drilled stone carries a permanent treatment and can be worn normally. A stone with colour added by HPHT or irradiation can give you a strong colour that GIA calls stable. The honest trade-offs are that the stone is no longer purely as nature made it and that some treatments bring care limits. If you want a stone with no treatments, say so at the start and ask for the report to confirm it.

A disclosed treatment is a fair thing to weigh; a hidden one is the problem. Fracture-filled and coated stones carry the clearest cautions, because the improvement itself can be damaged. If you are choosing a centre stone for a custom engagement ring, or a diamond for any custom piece, decide your position on treatments first, then compare stones. You can talk it through in a showroom.

Follow the treatment's limits. Fracture-filled diamonds can be damaged by steam, acid and ultrasonic cleaning, and if the filler turns dark it cannot be made colourless again. Irradiated diamonds may be more sensitive to the heat involved in repairs, and coated diamonds will scratch and wear off over time and can be damaged by heat and chemicals.

So tell any jeweller about a treatment before cleaning, resizing or repair work, and keep the paperwork that records it.