How Diamond Shape Changes What You Get

Brilliant cuts use triangular and kite facets radiating out from the center. They break incoming light into many small returns that disguise what is inside the stone. Round, oval, pear, marquise, cushion, and princess are all faceted this way.

Step cuts run long parallel planes in rows toward the girdle. Flashes are fewer and larger, with nothing between them to conceal an inclusion. Emerald and Asscher are the two in common use. Both ask for higher grades than a brilliant cut of equal size, and VS2 clarity with G color is a sensible floor. Above two carats, aim higher still.

A radiant has an eight-sided outline with trimmed corners, square or rectangular. Its pavilion is brilliant-faceted, so it scatters light the way a cushion or a princess does.

Round sets the baseline. It is the only outline GIA will grade for cut, and at 1 carat it covers about 33.2 square millimeters. Published facet counts differ by one, depending on the culet. A round has 57 facets when it finishes to a point and 58 when a small facet closes the tip.

An oval is longer end to end. Retail copy routinely puts it 10 to 15% larger than a round. The published measurements give it 33.3 square millimeters against the round’s 33.2. So the oval’s gain is its length, and the area is unchanged.

Both outlines are free of corners and points, which leaves the girdle as the only exposed edge on either. An oval shows a bow-tie, a dark band across the middle where light escapes through the pavilion. No grading document records it. Check it on video.

A pear is round at one end and pointed at the other. A marquise comes to a point at both ends. Both are long for their weight, and no outline here is longer than the marquise. It is also the only one in common use that covers more face-up area than a round, at about 36.9 square millimeters against 33.2, or roughly 11% more. A pear covers about 34.

On both, the point takes the damage, with the girdle thinning as it approaches. A prong formed to fit over the point protects it, and a bezel around the tip does it with more metal. Both outlines show a bow-tie.

A princess keeps its corners sharp. They fall close to the planes along which a diamond splits most readily, and no other outline in regular use has corners as exposed. Some cutters chamfer them, placing a facet so small it takes magnification to find. A radiant’s corners are cut off from the start. That removes the sharpest geometry, though a trimmed corner is still a corner.

At 1 carat, a princess measures about 5.5mm on a side, the figure with the most agreement across published charts, and covers about 30 square millimeters. A radiant covers about 32. Henry Grossbard designed the radiant in the late 1970s.

Elongated radiants are the same cut at a longer ratio. No gemological authority sets the point where one becomes the other.

A cushion has curved sides and either rounded or pointed corners, typically with four or eight kite facets running from girdle to culet. Adding extra rows of facets makes it a modified cushion brilliant. The trade calls that version crushed ice, and the smaller facets break light into finer pieces that disguise what is inside. Cushions are also cut deep, so more of the weight ends up below the girdle and out of sight. At 1 carat, a cushion covers about 29 square millimeters, the smallest face-up area of any brilliant cut here. Elongated cushions are a ratio band of that cut, with the threshold anywhere from about 1.10 to 1.30 depending on the seller.

An Asscher covers about 28.7 square millimeters at 1 carat, the smallest face-up area of any outline here. Cut deep like the cushion, it hides much of its weight under the girdle. An emerald covers about 31.8 against a round’s 33.2. Published charts disagree about the emerald more than about any other outline in common use.

An emerald also looks long on the hand, with a length-to-width ratio usually between 1.30 and 1.50. Even so, it covers less of that hand than a round does.

Both have corners. Trimming a corner reduces the exposure without removing it, on an emerald as much as on a radiant. An Asscher of 3 carats and above is usually set with double prongs at the corners.

Look at the culet first. On a stone above a carat, it is visible without a loupe, an open circle at the bottom where a modern stone comes to a point. Some Antwerp and Amsterdam stones from the early 1900s were finished without one, so its absence proves less than its presence.

Cutters of the period worked by gas lamp and candle. They were also paid to save weight, and weight is what drove the proportions. Rounding a stone by hand wasted material. The squarish outline survived until machinery made rounding cheap. Morse and Field patented steam-driven bruting machines in 1874 and 1876, the circular saw arrived around 1900, and the offcuts could then be recovered as smaller stones. By 1902 the trade could afford to cut for looks.

Electric light changed the cutting again. Once incandescent bulbs spread in the 1920s, cutters lengthened the lower girdle facets for more scintillation. The large facets of older styles throw fewer, bigger flashes than a modern round does.

The girdle on an antique stone can be very thin in places, so a setting that covers it all the way round is worth having. A stone cut to those proportions today has a thicker girdle. GOODSTONE sells old mine and old European cuts as lab-grown stones alongside natural ones. The large culet was a deliberate guard against chipping at the tip, and a modern stone finished to a point has given that up. 

One chart gives a 1-carat emerald 6.5 by 5.0mm. Another gives 7.0 by 4.5mm. Those are different stones at the same weight. Every outline carries a spread like it, narrower in most cases.

Carat measures weight, and weight can be put where it does not show. A thick girdle adds depth without adding width, and a deep pavilion does the same. Cutters call this spread. A 1-carat round can measure about 5.6mm across when cut deep and about 6.6mm when cut shallow. That gap is wider than the disagreement between any two published charts.

Hardness resists scratching, and toughness resists breaking. Diamond leads every material on hardness. On toughness, it measures about 5,000 along its cleavage planes and over 8,000 in its strongest direction, against 225,000 for nephrite and 120,000 for jadeite.

Diamond forms in the cubic system with four perfect cleavage directions, and a cutter splitting a stone gives it a sharp blow along one of them. An accidental knock along one of them does it just as well. Chips rarely come away as smooth planes. They step.

Seen face-up, a diamond’s facets break its body color into a mosaic of varying intensities. A busier pattern breaks it up more. GIA grades color with the stone table-down for that reason, looking through the pavilion to get past the facet pattern. So two stones graded alike can look different once set.