Diamond Cut Grades and Why They Matter More Than Any Other C

People say "I want a pear cut" when they mean shape. In grading language, shape is the outline of the stone seen from above. Cut is how well the stone was proportioned, faceted and finished. A pear can be cut beautifully or badly; so can a round.

Cut is also the only C a human controls. Nature or the growing chamber sets color and clarity. The cutter decides whether the finished stone sparkles or sulks, and often has to choose between maximum weight and maximum beauty.

GIA introduced its cut grading system for round brilliants in 2006, after years of research into how people perceive diamonds. The scale has five grades.

The overall grade blends several things: brightness, fire and scintillation (the appearance factors), plus weight ratio (is the stone heavy for its diameter?), durability (is the girdle too thin?), polish and symmetry. GIA notes that an overall Excellent requires polish and symmetry of Very Good or better. The AGS lab uses a separate 0 to 10 scale where 0 is the top, sometimes marketed as "Ideal."

• Brightness is white light reflected back to your eye from the inside and the surface. A well-cut stone looks bright even from across a room.
• Fire is white light split into flashes of color, most visible under spotlights or candlelight.
• Scintillation is the sparkle and the pattern of light and dark areas you see when the stone, the light or your hand moves.

A good cut balances all three. Some shapes lean one way by design: emerald cuts trade fire for broad flashes of light (the "hall of mirrors" look), while radiant and cushion cuts produce a busy, crushed-ice sparkle. Neither is better; they are different personalities. The quality question is whether the stone does its own job well.

Cutters work with a few key measurements. You do not need the numbers, but you should know what goes wrong when they are off.

The weight problem is real. A deep stone can weigh 1.00 carat and look like a 0.90. That is why carat weight alone is a poor guide to size, as our page on the 4Cs explains.

GIA does not give an overall cut grade to fancy shapes such as pear, oval, emerald, radiant, cushion, marquise or heart. Their reports list polish and symmetry, plus measurements, but there is no single agreed "ideal" for every outline. That puts more weight on your eyes and on the seller's honesty.

What to check by shape:

• Pear and oval: a soft, not inky, bow-tie; balanced shoulders; a pleasing length-to-width ratio.
• Radiant and cushion: even sparkle across the face with no dead center; corners that are clean and protected.
• Emerald: crisp, parallel steps; a bright central "hall of mirrors"; no dark, deep center.
• Marquise and heart: symmetry first, because any lopsidedness shows instantly.

The house rings cover all of these. The Radiant Lab Diamond Ring 7.98 ct, 14K Yellow Gold ($4,400) and the Emerald Lab Diamond Ring 4.98 ct, 14K Yellow Gold ($3,500) are good side-by-side lessons in two very different cutting styles.

Many elongated brilliants show a dark band across the middle, shaped like a bow-tie. It comes from areas where the pavilion angles send light out of the stone instead of back to your eye. Almost every oval, pear and marquise has some bow-tie; the question is how strong it is.

Photos often hide or exaggerate bow-ties depending on lighting, which is one more reason to see an elongated stone in person. Compare the Oval Lab Diamond Ring 2.47 ct, 14K White Gold ($3,600) under a spotlight and under daylight and you will see how much light changes the read.

On chains, pendants, bracelets and pavé bands, cut matters in a different way. Small diamonds, called melee, come in two main styles. Full-cut melee have 57 or 58 facets like a round brilliant and give a sharp, pinpoint sparkle. Single-cut melee have 17 or 18 facets and give broader, softer flashes. Both are real diamond cuts; full cuts look brighter, single cuts are often used in the smallest sizes.

What makes an iced piece look expensive is consistency: every stone cut to a similar standard, so the whole surface sparkles evenly. A few poorly cut or dark stones stand out against bright ones. When you look at an iced piece, scan it slowly under light and look for dead spots. Our pages on VVS grades and the clarity chart cover the other things to check on small stones.

Every GIA and IGI report lists polish (how smooth the facet surfaces are) and symmetry (how precisely the facets line up and meet). Both use the Excellent to Poor scale. Poor polish leaves the stone slightly hazy; poor symmetry can misdirect light and makes shapes look lopsided. For a center stone, look for Very Good or Excellent on both. Small differences between Excellent and Very Good are not visible to the eye.

A report gives you the numbers behind the look. For a round diamond, read these lines in order:

For fancy shapes there is no overall cut grade, so measurements carry more weight. Divide the length by the width to get the ratio, compare the face-up size with other stones of the same weight, and check polish and symmetry. A stone that is heavy but measures small is spending its carats where you cannot see them.

Cutting is a trade-off between beauty and weight. Rough diamond is priced by weight, and so are finished stones, so a cutter is always tempted to keep extra weight even if it costs some brilliance. Cutting for top light performance usually means grinding away more rough, which means a smaller finished stone from the same crystal. That lost weight is why an Excellent cut costs more than a Good cut of the same carat, color and clarity.

The same pressure explains "magic sizes." Prices step up at round numbers such as 1.00, 1.50 and 2.00 carats, so some stones are cut slightly deep or with a thick girdle just to cross the line. A 1.01 ct stone that faces up like a 0.95 is a common result. Judge the spread, not just the number on the tag.

1. Look from the top, at arm's length. A well-cut stone looks bright edge to edge, with no dull center and no grey ring.
2. Rock it slowly. You should see sparkle move across the face, not a few fixed bright spots.
3. Change the light. Check under spotlights for fire and near a window for brightness.
4. Compare side by side. Put two stones of similar weight next to each other. The better cut usually looks larger and whiter.
5. Check the spread. Measure or ask for the diameter. A stone that measures small for its weight is carrying hidden depth.

You can do all of this with the house rings at our Miami showroom; directions are on the visit page. Then read the color chart, VS1 clarity explained and the certificates guide to round out the picture.