Diamonds are graded on four characteristics, developed by the Gemological Institute of America and now used universally. Understanding them is what lets you compare two stones and know whether a price difference is justified.
They are not equally important, and knowing which to prioritise is where most of the value in this sits.
Cut
Cut is the most important of the four for how a stone actually looks, and it is the one most often misunderstood.
Cut does not mean shape. Round, oval, princess and emerald are shapes. Cut refers to the proportions, symmetry and polish of the facets, and how well they return light to the eye.
A poorly cut stone leaks light through the bottom rather than reflecting it back. It can be flawless and colourless and still look dull. A well cut stone of lower colour and clarity grades will usually outshine it.
Grades run from Excellent through Very Good, Good, Fair and Poor. If you are trading grades against budget, this is the last one to compromise.
Colour
Colour grading measures the absence of colour. The scale runs from D, meaning completely colourless, through to Z, showing noticeable light yellow or brown.
D through F are colourless. G through J are near colourless and appear white to most eyes, particularly once set. K onwards shows warmth that becomes progressively more visible.
The practical point is that adjacent grades are very difficult to distinguish without comparing stones side by side under controlled lighting. The price difference between them can be substantial. Many buyers get better value dropping a grade or two into the near colourless range than paying the premium for D.
Setting metal matters here too. A warmer stone can look whiter in yellow gold than in platinum or white gold.
Clarity
Clarity measures internal features, called inclusions, and surface features, called blemishes. The scale runs from Flawless and Internally Flawless, through Very Very Slightly Included at VVS1 and VVS2, Very Slightly Included at VS1 and VS2, Slightly Included at SI1 and SI2, and Included at I1 to I3.
The term worth knowing is eye clean, meaning no inclusions visible to the unaided eye at normal viewing distance. Many VS and even SI stones are eye clean, and to anyone looking at the ring they are indistinguishable from a flawless stone.
Since clarity grading is done under 10x magnification, paying a large premium for a grade nobody will ever perceive is a common and avoidable overspend. What matters is where the inclusions sit, not just how many there are. An inclusion under the table is more visible than one near the girdle where a setting may cover it.
Carat
Carat is weight, not size. One carat is 0.2 grams, or 200 milligrams, divided into 100 points, so a 50 point stone is half a carat.
Two things are worth knowing. Price rises disproportionately at round numbers, because a 1.00ct stone commands a noticeable premium over a 0.95ct one that looks essentially identical. Buying slightly under a round weight is one of the more reliable ways to get more stone for the money.
And weight is not the same as spread. A deeply cut stone carries weight in its depth where it cannot be seen, so it looks smaller face up than a well proportioned stone of the same carat. This is another reason cut deserves priority.
How to prioritise
For most buyers, the order that produces the best looking stone for a given budget is cut first, then carat, then colour, then clarity.
Get the cut right, choose a weight slightly below a round number, settle into the near colourless range, and buy the lowest clarity grade that is still eye clean. That combination consistently outperforms chasing top grades across the board.
Frequently asked questions
Which of the 4Cs matters most?
Cut, because it determines how the stone returns light. A poorly cut diamond looks dull regardless of its other grades.
What does eye clean mean?
No inclusions visible to the unaided eye at normal viewing distance. Many VS and SI stones qualify.
What is the best colour grade to buy?
D through F are colourless, but G through J appear white to most eyes at lower cost. The right grade depends on budget and setting metal.
How big is a one carat diamond?
One carat is a weight, 0.2 grams, not a size. Visible size depends on how the stone is cut, since a deep cut hides weight below the surface.
Is a flawless diamond worth the premium?
Clarity is graded under 10x magnification. If a lower graded stone is eye clean, the visible difference is nil while the price difference is not.
Our diamonds
We hold certified lab grown and natural stones. See the range, read about lab grown versus natural, or visit the showroom at 124 Peterborough Street, Christchurch.