A certificate, sometimes called a grading report, is produced by a gemmological laboratory that has examined the stone under controlled lighting and magnification. It is not a valuation and it is not a guarantee of price. It is a description, and its usefulness comes entirely from the independence of whoever wrote it.

• GIA: The Gemological Institute of America created the modern colour and clarity scales and is the strictest of the major laboratories. A GIA report is the reference point the rest of the trade is measured against.
• IGI: The International Gemological Institute grades both mined and laboratory-grown stones and is the most widely used laboratory for lab-grown diamonds.
• HRD: The Antwerp laboratory, long established in the European trade.

Grading involves human judgement, so laboratories are not perfectly interchangeable. A stone graded G by one may be graded H by another. This is why comparing two diamonds on paper alone is unreliable unless both were graded by the same laboratory.

What the report tells you

• The four Cs: carat weight, colour grade, clarity grade and cut grade, each on a published scale.
• Measurements and proportions: the actual dimensions, table and depth percentages, and the angles that determine how the stone handles light.
• Fluorescence: whether the diamond glows under ultraviolet light, and how strongly.
• A clarity plot: a diagram showing where the inclusions sit, which is how a specific stone is identified.
• Origin: whether the diamond was formed in the earth or grown in a laboratory. Every reputable report states this plainly.
• A laser inscription: on most modern stones, the report number is inscribed on the girdle in letters too small to see without magnification.

Why we insist on it

Colour and clarity are the two grades that move the price most, and they are the two hardest to judge by eye. A single grade of difference can be a significant sum on a larger stone, and almost nobody can see it across a counter. The certificate is what makes that difference checkable rather than a matter of trust.

You are welcome to ask for the report number of any diamond before you commit, and to verify it yourself on the laboratory's own website. We would rather you did.