Four words everyone repeats, and only one of them is decided by a human.

The 4Cs were created by GIA, and they remain the most useful framework in diamond buying. But they are usually presented as four equal considerations, which is misleading — they are not equally important, and the order most guides put them in is close to backwards.

Carat: weight, not size

A carat is 0.2 grams. It measures weight, not dimensions — which is why two one-carat stones can look noticeably different in a ring. A deep-cut stone hides weight in its depth where nobody sees it; a well-proportioned stone spreads it across the face.

Price rises steeply rather than proportionally, because larger rough is rarer. A 1.00 ct stone costs considerably more than two 0.50 ct stones of the same quality.

One practical trick: stones just under the round numbers — 0.90 rather than 1.00, 1.40 rather than 1.50 — cost meaningfully less and are almost impossible to distinguish by eye.

Color: less color, more white

Graded D through Z, where D is completely colorless and the scale runs toward faint yellow or brown.

Our house standard is H–I or better — the point at which a stone reads white against skin and where additional spend stops being visible.

One thing worth knowing: the metal affects the apparent color. A faintly warm stone looks whiter in yellow gold than in platinum, because there is nothing cold beside it for comparison.

Clarity: what you can and cannot see

Clarity describes internal inclusions and surface blemishes, graded under 10x magnification — which is the key to understanding it. The grade describes what a gemologist sees with a loupe, not what you see across a dinner table.

• FL–IF — flawless and internally flawless. Extremely rare, and the premium is not visible to the eye.
• VVS1–VVS2 — very, very slightly included. Difficult to find even under magnification.
• VS1–VS2 — very slightly included. Eye-clean. Our standard.
• SI1–SI2 — slightly included. Sometimes eye-clean, sometimes not. Must be seen.
• I1–I3 — included. Visible to the naked eye.

VS2 is the sensible floor, and it is our house standard on every stone. Paying past it buys a better certificate rather than a better-looking diamond.

Cut: the only C humans control

Carat, color, and clarity were decided in the ground. Cut is decided by a person, and it is the one that determines how the diamond actually behaves in light.

A well-cut stone returns light through the top as brilliance and fire. A poorly cut one — too deep or too shallow — leaks light out of the bottom and looks dull no matter how good its other grades are.

This is where to spend. A well-cut 0.90 ct outperforms a poorly cut 1.10 ct, and looks larger doing it. We carry Excellent and Very Good only.

Note that a formal cut grade is assigned to round brilliants. For fancy shapes, judge proportions and symmetry — or judge the stone in person.

Working specs by piece type

What we would actually recommend, rather than the theoretical ideal:

Piece-specific guidance is in how to buy a diamond tennis bracelet and how much should an engagement ring cost.

The fifth C: certification

Any center stone of 1.00 carat or above should carry an independent report from GIA or IGI. Verify the report number on the laboratory's own website, and check that the laser inscription on the girdle matches. Never accept an in-house "certificate" from the seller's own laboratory.

Note that GIA and IGI do not grade to identical tolerances, so comparing stones across laboratories on stated grades alone will mislead you.

Care

Diamond is the hardest natural substance, but the setting is not. Most diamond loss starts with a loose prong rather than with the stone.

• Clean with warm water, a drop of mild dish soap, and a soft brush. The film that dulls a diamond is skin oil and hand lotion, not damage.
• Have prongs checked once a year. Our annual inspection is complimentary.
• Take rings off for the gym, gardening, and cleaning.
• Store separately from pearls — diamond will scratch nacre.

Why ours feel different

We buy through cutters in the major diamond centers, directly, which is how we can hold a stated standard rather than an approximate one: VS2 clarity or better, H–I color or better, Excellent or Very Good cut, on every stone in the collection.

Accent stones are matched to the center for color and cut quality — the detail that separates a piece that looks considered from one that looks assembled, and the one nobody mentions until you see the two side by side.

Read lab-grown vs. natural diamonds and 14K vs 18K gold vs platinum. Browse diamond jewelry and engagement rings.

Elegantly modern. Forever classic.

ISAAC WESTMAN® · New York · Founded 1982