Diamond Cut Grades Explained: Excellent to Poor

Cut is often misunderstood. People think it means shape. It does not. Shape is the outline, such as round or oval. Cut is the quality of the work: the angles, proportions and finish that decide how light travels through the stone. It is the C that most affects how alive a diamond looks.

What a good cut does

Light enters the top of the diamond, bounces between the facets inside and returns to your eye. A well-cut diamond sends most of that light back up. A poorly cut one lets light leak out of the sides or bottom, so it can look dull or glassy even if its colour and clarity are excellent.

Three effects come from this:

• Brilliance: the white light reflected from the stone.
• Fire: flashes of rainbow colour.
• Scintillation: the sparkle and pattern of light and dark as the stone or the light moves.

The grading scale

For standard round brilliant diamonds, cut is graded on a scale: Excellent, Very Good, Good, Fair and Poor. The grade reflects the stone's proportions, symmetry and polish. Fancy shapes such as oval or pear do not always have a cut grade on every laboratory's report, so they are judged by visual inspection and by their proportions and finish.

What drives the grade

• Table size: the size of the flat top facet relative to the width.
• Depth: the total height relative to the width. Too deep or too shallow leaks light.
• Crown and pavilion angles: the angles of the upper and lower facets, which must work together.
• Girdle thickness: very thin edges chip, very thick ones add weight without beauty.
• Symmetry and polish: how exactly the facets match and how smooth the surfaces are. See diamond polish and symmetry.

Which grade should you buy?

For round diamonds in engagement rings and solitaires, many buyers choose Excellent or Very Good, since they give the most light for the money. For small stones in a pavé setting, individual grading is not practical, and the overall quality of the cutting is what counts.

Cut over carat

Many experts suggest putting budget into cut before size. A smaller, well-cut diamond often looks livelier than a larger one that is poorly cut. See carat weight versus diamond size.

How to judge cut in person

1. Look at the stone under different lights, shop light and daylight.
2. Tilt it and watch for even sparkle across the face, without dead or dark areas.
3. Compare two stones side by side.
4. Read the cut grade on the certificate.

Cut and the certificate

The grade appears on the report. Our walkthrough is in how to read a diamond certificate step by step.

Common questions

Can a diamond have a perfect colour and clarity but a poor look? Yes. If the cut is poor, light leaks and the stone can look dull despite high grades on the other Cs.

Does cut affect size? It can. A deep stone carries more of its weight below the surface and looks smaller from above.

Is ideal cut a grade? "Ideal" is a term some sellers use for very well-proportioned stones. Prefer the grade on the laboratory report.

Ask to compare

A comparison teaches faster than any chart. Book an appointment to see well-cut and average-cut diamonds together. You can also look through our diamond earrings, where each stone's details are listed.