A grading certificate is how the diamond trade compares one stone with another without either party taking the other's word for it. It is genuinely useful. It is also narrower than most people assume, and knowing where its edges are will stop you paying for the wrong things.
What IGI is
The International Gemmological Institute is an independent laboratory. It does not sell diamonds and has no stake in what you pay for one. Its job is to look at a finished stone and describe it against a published scale, so that two stones graded in the same place can be lined up against each other honestly.
Where a piece of ours has a certified centre stone, it is marked as such on the product page, and the certificate goes in the box with it.
The four things it grades
Carat weight. Straightforwardly a measurement. One carat is a fifth of a gram. This is the one number on the report that cannot be argued with.
Colour. Graded D through Z, where D is colourless and the letters walk gradually toward a visible warm tint. The gaps between neighbouring grades are subtle. Once a stone is set in yellow gold, the metal influences how the colour reads anyway.
Clarity. A description of the tiny internal features almost every diamond has, running from flawless down through the various "included" grades. The important question is not how high the grade is but whether anything is visible to you at normal viewing distance. Plenty of stones a grade or two down from the top look identical in the hand and cost noticeably less.
Cut. For round brilliants, a graded assessment of how well the proportions return light. This is the one we would tell you to protect first. Cut is what makes a diamond look alive, and a poorly cut stone with excellent colour and clarity will still look dull. Fancy shapes such as ovals, pears and emeralds are not given an overall cut grade in the same way, which is exactly why they need looking at rather than reading about.
What the certificate does not cover
It grades the stone. It says nothing about the piece the stone is set into.
It will not tell you whether the setting is well made, whether the prongs are substantial enough to hold securely for years, whether the metal is finished on the underside where nobody looks, or whether the proportions of the whole thing work on a hand. Those are the parts that determine whether you enjoy wearing it, and none of them appear on the report.
It also says nothing about value. A certificate describes; it does not price. Two stones with matching reports can be priced quite differently and both prices can be fair, depending on what surrounds them.
And it does not settle the question of whether you like the stone. That still needs your eyes.
Why grades are worth comparing carefully
Laboratories are not perfectly interchangeable. Grading involves human judgement at the margins, and there is long-running trade discussion about how consistently different labs apply the same scales.
The practical response is simple. Compare like with like. Two stones graded by the same laboratory can be lined up against each other with confidence. Two stones graded by different laboratories, or one graded and one not, cannot be compared on paper alone, and anyone telling you otherwise is selling.
How to actually use one
Read the cut grade first, then the clarity, then the colour, then the carat weight. That is roughly the reverse of the order most people read them in, and much closer to the order in which they affect what you see.
Then stop reading and look at the stone.
If you want us to walk through a particular report with you before you order, write to us. We would rather spend the time up front than have you buy something you are unsure about.