Diamond Cut: Proportions and Light

When I compare diamonds, I want to know how they look when they move. The carat weight matters, but so does what happens when the stone catches the light.

That is why I put so much emphasis on cut. Before deciding how large a diamond should be, understand how well it has been cut.

What are you looking at?

Brightness is the return of white light. Fire appears as flashes of colour. Scintillation describes the sparkle and changing pattern as the diamond, light or viewer moves.

A diamond's outline may catch your attention first. Give yourself time to notice what happens within it.

Measurements deserve more attention than they get

Carat tells you weight. Measurements tell you the stone's dimensions. Proportions describe how its parts relate to one another.

These measurements belong in the same conversation. Picking one attractive number and ignoring the others can lead you away from a better stone.

The temptation to choose the biggest outline

If you are new to comparing diamonds, the one that looks largest can seem like the obvious choice. But a wider outline does not establish better cut quality.

A shallow stone may offer more spread for its weight while compromising light performance. At the other extreme, excess depth or a very thick girdle can carry weight that contributes little to its apparent size.

At every carat weight, look for proportions that work well together. You do not need to give up that standard because you have a particular size in mind.

My advice is to ask the question directly: “What am I gaining—and what am I giving up—with this diamond?”

What does an Excellent cut grade mean?

For natural standard round-brilliant diamonds in the D–Z colour range, GIA's cut scale has five grades: Excellent, Very Good, Good, Fair and Poor. Excellent is its highest grade; Ideal is not an additional grade above it in that scale.

GIA's current assessments for qualifying D–Z lab-grown diamonds use the overall descriptions Premium and Standard. Check the laboratory and report date before comparing the terminology on lab-grown reports. GIA's reporting update.

A description such as Ideal needs context. Ask who assigned it and what criteria they used. Do not treat different laboratories' terms or a seller's label as interchangeable.

GIA also offers an AGS Ideal Report as a separate light-performance supplement for eligible diamonds.

There is more than one successful combination

There is no single proportion recipe that guarantees the most beautiful diamond. Crown and pavilion angles, table size and the other features work together. GIA's research recognises multiple combinations within a cut grade.

Use the report to inform your comparison, then look at the individual diamond. Even stones with the same grade can appeal to you differently.

For an oval, pear, emerald cut or another shape, ask for an assessment appropriate to that shape. A round-brilliant checklist should not be applied indiscriminately.

How I would approach your shortlist

Bring the report and imagery for each stone, together with the ring design you have in mind. We can compare the dimensions and discuss what the proportions mean for that particular choice.

Ask to see the diamond in more than one lighting condition. Compare it in motion, and ask what the price includes. If the shortlist changes, keep the qualities that matter to you in view.

The aim is to choose a diamond you enjoy looking at, in a ring that feels right for the person wearing it.

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Sources: GIA cut overview, GIA cut grading.

Further reading: GIA cut research, GIA laboratory-grown reporting update, AGS Ideal Report by GIA.