How a Gemstone Is Cut

A gemstone in a parcel of rough does not look like the stone in a ring. Cutting is the work that opens it. The mineral stays the same. The outline, the brilliance and a good deal of the weight do not.

Rough is not a promise

A crystal may be whole, water-worn, cracked, or already in pieces from the mine. The cutter looks for colour, for directions that will split, and for flaws that would break out of a facet. A large rough can yield a small clean stone. A smaller rough can yield a better one. Carat weight of the rough is not the carat weight you will wear.

Some stones have a direction of cleavage, a plane where the crystal prefers to split. Diamond is the famous case. A cutter respects that plane. Ignoring it is how stones break.

Sawing and the preform

The rough is sawn or ground to a first shape, the preform. For a round brilliant that shape is a cone-like outline. For a cabochon, a smooth dome with no facets, the outline is a mound. Cabochons are the usual cut for star sapphires, cat’s-eye, and many opaque stones, because the effect sits on a curved surface. Facets would break the star or the eye into pieces.

The preform decides the depth. A stone cut too shallow can look bright face-up and lifeless when you tilt it, or it can look dark if too deep. There is no single diagram that fits ruby, emerald and tourmaline. Each mineral bends light differently. A “perfect cut” chart made for diamond is the wrong ruler for an emerald.

Facets

Facets are flat windows. They are cut on a lap charged with a powder harder than the stone, then polished with a finer agent. Diamond will cut diamond. Corundum needs a harder powder than quartz. The Mohs scale is a rough order of scratch hardness. It is not a measure of how easily a stone shatters. Jade is not very hard and is remarkably tough. Diamond is the hardest and can still cleave.

The arrangement of facets, brilliant, step, mixed, rose, is a style. A step cut, with long parallel facets, is often used for emerald. It does not make the stone an emerald. Glass and other green stones can be step-cut too.

Weight is lost on purpose

Cutting removes material. That loss is normal. A seller who implies that the polished weight should equal the rough weight does not understand the work, or is hoping you do not. The number that matters on the bill is the weight of the stone you are buying, in carats, after cutting.

What the eye can check

Look at the outline. Is it even, or did the cutter leave a lump to save weight? Look at the facets. Are they scratched, or do they meet cleanly? Look through the table, the large top facet. A window, a see-through pale patch, can mean the stone is shallow. Dark extinction can mean it is deep, or that the colour is strongly zoned.

None of these glances replaces a laboratory report when the stone is costly or the name is specific. Cutting is a craft. It can reveal a fine colour. It cannot turn one mineral into another.