Cutting & lapidary

Before any cut, a planner decides what a rough stone should become. Modern planning begins with scanning. Sarine Technologies, established in 1988 and incorporated in Israel, released its Galaxy inclusion mapping system in 2009 and describes it in its 2025 annual report as the first rough diamond inclusion mapping solution. Planning software takes that inclusion map together with a geometric analysis of the stone's exterior: Sarine states that its Advisor program calculates millions of options, predicting the clarity of each proposed polished stone as well as its weight and shape.

The choices are commercial as much as technical. In 1919 Tolkowsky put the weight removed by bruting and polishing at 52% of the rough even in the most favorable cases. A planner may divide one crystal into two stones, choose a fancy shape that follows the rough, or place an inclusion near the edge where a setting can hide it. GIA notes that diamonds at or just above a magic size such as 0.50 ct or 1.00 ct usually cost more per carat than slightly lighter stones, so a plan that finishes just above a threshold can be worth more than one with better proportions just below it. With colored stones, the cutter orients the rough so the best color faces up.

Diamond has perfect cleavage on four equivalent sets of {111} planes, the planes of its octahedral crystal faces. In cleaving, the cutter scores a groove along one of them and splits the stone with a blade and a sharp blow. The Royal Collection Trust records the best known example: the 3,106 ct Cullinan went to Asschers of Amsterdam, four days went into preparing the groove, and the first blow broke the knife rather than the diamond. Three men then worked 14 hours a day for eight months to produce nine numbered stones, along with 96 or 97 small brilliants depending on the account.

Where no cleavage plane suits the plan, the stone is sawn. A thin disc of steel, copper or phosphor bronze, charged along its rim with diamond grit, scratches its way through. Lasers now do much of this work, and Sarine's Quazer machines saw and shape with a 532 nm green laser. Bruting, or girdling, rounds the outline by grinding one diamond against another; GIA dates the bruting machine to 1873. Polishing follows on a scaife, a cast-iron wheel charged with diamond powder, since only diamond powder will polish diamond. Colored stones are cut on a faceting machine, the stone glued to a dop stick held in a handpiece that controls the angle.

The round brilliant is the only shape GIA gives a cut grade, and GIA describes it as the majority of diamonds sold. The CIBJO Diamond Book of 2024 sets out its facet arrangement: one table, eight bezels, eight stars and 16 upper girdle facets above, eight pavilion mains and 16 lower girdle facets below, plus the culet, for 57 or 58 facets in all. Counting the culet with the pavilion splits them 33 and 25. A modified brilliant keeps the radiating brilliant pattern on a different outline.

The oval is an elongated round. GIA's 2024 study of fancy shapes covered ovals with length-to-width ratios from 1.2 to 1.7, pears from 1.3 to 1.8 and marquises from 1.5 to 2.5. The pear has one rounded end and one point; the marquise, also called the navette, is pointed at both ends. The heart needs precise symmetry, and GIA advises 0.50 ct or more for the outline to read clearly. The trillion is a triangular brilliant often used for side stones.

Elongated brilliants tend to show a bow tie, a band of dark contrast seen through the table. GIA found that it ranges from slight to thick and that large, dark, persistent bow ties count against a stone.

The princess is a square or rectangular brilliant shaped like an inverted pyramid, with much of its weight in the pavilion. GIA reports call it a square modified brilliant, date its creation to 1981 and name Betzalel Ambar and Israel Itskowitz as its makers. It has no culet, so the pavilion point can chip unless the setting guards it, and GIA notes that its facet count and arrangement vary widely. The cushion has a square or rectangular outline with rounded corners and comes in brilliant and modified brilliant styles.

Step cuts use rows of long facets parallel to the girdle. The emerald cut is a rectangle with cropped corners; GIA says it was developed for emeralds to reduce pressure during cutting and prevent chipping, and its long open facets show clarity and color rather than sparkle. Joseph Asscher worked out the geometry of a steep square cut in 1902 and patented it, which the firm describes as the first patented diamond; Royal Asscher's later signature cuts carry 74 facets. The radiant puts brilliant-style faceting, 70 facets in all, on a cut-cornered rectangle or square. The briolette is a drop covered in triangular facets with no table, a shape Tolkowsky recorded in 1919 as generally ascribed to Lodewyk van Berquem.

Before the modern brilliant, cutters worked without precise angle calculations or mechanical bruting. Tolkowsky's history of 1919 dates the rose cut to about the middle of the sixteenth century and describes a flat base under a domed crown. A study in Gems & Gemology in 2020 gives the standard form as six triangular facets at the center of the dome over a further layer of six, 12 or 18, which is what makes the Antwerp, half-Dutch and full-Dutch roses 12, 18 and 24 facets. With no pavilion a rose gives soft flashes rather than fire, and it suits flat rough and cleavage fragments.

The old mine cut has a soft squarish outline and 58 facets, with a small table, a high crown, short lower halves and a large culet. GIA traces its origins to the early 1700s and calls it one of the commonest cuts from the early to the late 1800s. The old European cut is round with a smaller culet. GIA identifies a classic example by a table of 53% or less, a crown angle of 40° or more, lower halves of 60% or less and a culet slightly large or larger, and does not give it a cut grade. Most older round brilliants were cut between the 1870s and the 1940s.

GIA sorts fashioned stones by facet arrangement. Brilliant styles use triangular and kite-shaped facets, with kite-shaped crown mains. Step cuts have four-sided facets whose upper and lower edges run nearly parallel, and they usually end in a keel line rather than a culet; the emerald cut and the narrow baguette belong here. A mixed cut uses both styles in one stone, commonly a brilliant crown over a step-cut pavilion, as on many ovals and cushions. Gems & Gemology reported in 1982 that the cutter Basil Watermeyer had introduced a split-facet technique that largely removes the bow tie, applied among other places to a Barion emerald cut.

Bernd Munsteiner, born in 1943 and working at Idar-Oberstein, invented the fantasy cut: grooves, optic dishes and concave facets ground into the back of a stone. His Dom Pedro aquamarine of 1993, 10,363 ct, is at the Smithsonian. Precision cutting means faceting to angles modeled in software beforehand rather than following the rough.

A cabochon has a smooth convex top over a flat or domed base. Stones showing asterism or chatoyancy are cut this way because the fine needles that cause them would not show in a faceted stone, and the base is cut parallel to the needles so the star or eye sits evenly on the dome.

A faceted gem looks bright because of total internal reflection. Light entering through the crown must strike the pavilion at angles steeper than the material's critical angle, which Tolkowsky calculated at 24°26′ for diamond, so that it reflects back up instead of leaking out. GIA calls the result of a pavilion cut too shallow windowing: light passes through and the viewer sees what lies behind the stone.

In 1919 Marcel Tolkowsky published Diamond Design, which worked out a best section for the round brilliant. Taking the girdle diameter as 100, he gave a table of 53.0, a thickness above the girdle of 16.2 and below it of 43.1, a pavilion angle of 40°45′ and a crown angle of 34°30′. He traced rays through one or two internal reflections and set dispersion inside the stone aside. GIA introduced its own cut grading system in 2005 and began putting cut grades on round brilliant reports in 2006; its research concluded that many different proportion sets produce top-grade stones, a wider range than trade practice suggested. The American Gem Society Laboratories, founded in 1996, had pressed for independent cut grading earlier.

A Hearts and Arrows diamond shows eight symmetrical arrows face up and eight hearts table down under a viewer.

Colored stones are often first faceted close to where they are mined, by cutters whose priority is weight; the trade calls the results native cuts. GIA's guidance on colored stone cut quality describes what goes wrong. Pavilion facets cut too shallow for the material produce a window, so the viewer sees straight through the stone, and a girdle reflected under the table gives a fisheye. Dealers buy such stones and have them recut for better symmetry and light return, accepting a loss of weight when the higher price per carat makes up for it. A recut stone no longer matches its old grading report, because its weight and proportions have changed.

For historic diamonds, recutting can erase original work. The Koh-i-Noor was shown at the Great Exhibition from 1 May to 15 October 1851 and disappointed its audience; The Times reported that many visitors could hardly believe it was anything but a piece of common glass. Prince Albert had it recut, and the work was done in London in 1852 by the crown jewelers Garrards. Historic Royal Palaces records the result at 105.6 ct, recut to improve its brilliance and to suit European taste, and set in the Crown of Queen Elizabeth the Queen Mother in 1937.

Gem carving is far older than faceting. An intaglio has its design cut into the surface, in sunken relief, so that pressed into wax or clay it leaves a raised image; the British Museum notes that such gems were often used as seals as well as being luxury objects. A cameo is the reverse, with the design raised in relief. The Metropolitan Museum dates the change to about the fifth century BCE, when Greek carvers began engraving stones in projected relief, and credits the Hellenistic Greeks with being the first to excel at it. Carvers favor banded stones such as sardonyx, onyx and agate, playing a pale layer against a dark ground. The cutting was done not with the point of the drill but by using the drill to rub abrasive powders into the stone.

Two Roman pieces show the scale reached. The Gemma Augustea in Vienna, 19 × 23 cm, was cut from a two-layer Arabian sardonyx between 9 and 12 CE. The Grand Camée de France in Paris, 31 × 26.5 cm, was cut from a five-layer sardonyx in the first quarter of the first century CE.

Idar-Oberstein has worked agate from the Nahe valley since the 1400s. Nephrite is shaped by abrasion: the Liangzhu culture of about 3300 to 2250 BCE ground it into disks (bi) and tubes (cong) placed with the dead.