How to Read a Gem Certificate

A gem certificate is not a guarantee of value. It is a set of measurements and opinions from people who looked at the stone with instruments you do not have. Knowing which parts are measured and which are opinion is most of what you need.

Identity comes first

The top of the report says what the stone is. Natural corundum. Synthetic corundum. Glass filled corundum, which is a different thing wearing the same name. Everything below this line assumes this line is right, so read it before anything else.

Treatment is where the money is

Heat treatment is routine. Most sapphire on the market has been heated, and there is nothing wrong with a heated stone honestly sold. The problem is that an unheated stone of the same size and colour can be worth several times more, which gives everyone in the chain a reason to be vague.

A report will say no indications of heating, or it will describe the heating it found. If a seller says unheated and cannot produce a report that says so, you are being asked to pay an unheated price for a claim.

Watch for the heavier treatments as well. Beryllium diffusion changes colour right through the stone. Lead glass filling holds a fractured ruby together and will not survive a jeweller’s torch. Both are disclosed on any real report and both should be reflected in the price.

Origin is an opinion

Laboratories work out origin from inclusions and trace element chemistry, then compare against reference stones. It is careful work and it is still an inference. Two laboratories occasionally disagree about the same stone.

Origin matters commercially because Kashmir, Burmese and Ceylon material carry premiums. If you are paying one of those premiums, get the origin from SSEF or Gubelin rather than from a seller’s description.

What is missing from the report

No sapphire report grades colour the way a diamond report grades colour. GRS puts trade terms like royal blue on a report, and the others largely do not. Nothing grades cut quality, which decides how much light comes back out of the stone.

This is why two stones with near identical reports can differ in price by several times. The report tells you what the stone is. It does not tell you whether it is beautiful, and beauty is most of the price.

Check the number yourself

Every major laboratory runs a public report lookup. Take the number, enter it on the laboratory’s own website, and confirm the weight and measurements match the stone you are being offered. It costs a minute and it catches the oldest trick in the trade, which is a real report attached to a different stone.

If a seller will not give you the report number before payment, you already have your answer.