The grades
| Grade | What is happening to the light | Worth buying? |
|---|---|---|
| Excellent | Almost everything entering the top returns to your eye | Yes, start here |
| Very Good | Slight loss, visually very close to Excellent | Yes, good value |
| Good | Noticeable leakage, less life in the stone | Only on a tight budget |
| Fair | Substantial leakage, looks flat | No |
| Poor | Most light lost through the sides or bottom | No |
What cut actually controls
Light enters through the flat top of a diamond, bounces off the angled facets underneath, and comes back up at you. That is the whole trick. Whether it works depends on angles measured to fractions of a degree.
Cut a stone too deep and light entering the top hits the pavilion at an angle that lets it escape out the bottom, so the middle of the stone goes dark. Cut it too shallow and light leaks out of the sides, which produces a flat, glassy look sometimes called a fisheye. Neither can be fixed afterwards.
Why cut beats colour and clarity
Colour and clarity are properties of a rock that formed a billion years ago. They set an upper limit on the stone but they do not determine how it behaves in a room with light in it.
Put an Excellent cut J colour VS2 next to a Poor cut E colour VVS1 and almost everyone picks the first one, despite the second having far better paperwork and a far higher price. Brilliance is what people actually see.
Cut also controls apparent size
A deep cut stone hides weight in its belly, below the girdle where nobody looks. Two one carat stones can differ by half a millimetre or more across the top depending on how they were cut, which is a visible difference on a hand.
So a well cut stone is not only brighter, it looks bigger for the same carat weight. See carat to millimetre conversion for the expected diameters.
Polish and symmetry
Two smaller grades sit alongside cut on the certificate. Polish covers surface finish and symmetry covers whether the facets line up as they should. Both are graded Excellent through Poor. Very Good or better on each is fine, and chasing Excellent on all three adds price without adding much you can see.
Not sure what you are looking at?
Photograph the stone and the app gives you a visual read on the 4Cs, an estimated value range, and what to check next. Free on iOS and Android.
Frequently asked questions
Is cut the same as shape?
What is an ideal cut?
Why do only round diamonds get a cut grade?
Should I ever buy below Very Good?
Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.