How Can Steel Blades Cut Diamond?
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Crystallographic Structure
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Despite the hardness of a diamond in general, it can be cut by materials that are not as hard. This is possible only because the hardness in diamond varies widely by direction, due to its underlying crystallographic structure.
Along one plane in its structure, diamond has a plane with a minimum number of covalent bonds (see red dash in diagram), called a cleavage plane. Diamond cutters use this plane to cleave some diamond stones, prior to faceting.
History
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Cleaving is the most ancient diamond-cutting process. Because only one plane was available for cutting, the type of cuts, and therefore the overall gem shape, was limited.
To cleave diamonds, the diamond cutter would place a chisel at a point of weakness in the stone and tap it with a mallet, causing the diamond to split into two pieces with perfectly flat surfaces.
In the fifteenth century, the scaif was developed. A scaif is a polishing wheel with a lap, or disk, lubricated with oil and diamond dust. A diamond cutter can use a scaif to polish a diamond held in a dop, a padded diamond holder that protects the diamond by revealing only the side currently being polished. The scaif made available a much wider range of possible gem shapes, revealing facet angles that brought out more shine and sparkle.
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Modern Cleaving
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Before cleaving with a steel blade, the diamond is notched with the fragment of another diamond, to hold the steel cleaving blade in place. Sometimes, this notch is made with a laser. The cleaving blade is then placed into the notch, and then hit with a metal bar to spread the notch and cleave the diamond.
The diamond is cemented firmly to a support in such a direction that the cut to be made is vertical, so that the force of the blow will be driven down into the wooden shaft of the dop.
Modern Faceting: Sawing, Bruiting and Polishing
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After a diamond is cleaved, it may still not be a manageable size for faceting, since cleaving is possible along only one plane. To further reduce its size along other planes, the diamond cutter uses a phosphor-bronze rotating saw with a blade embedded with oriented diamond fragments. Even a saw without diamond embedding can cut planes other than the cleavage plane, owing to the high rate of rotation, something that was not achievable centuries ago. Diamond cutters may use a laser to saw the diamond, but this takes several hours.
After sawing, bruiting is performed, which is the first step in cutting the shape of the diamond. It shapes the girdle, or outermost edge. This can be performed by rubbing two diamonds together, cemented firmly in dops.
After bruiting, a lathe is used to cut the detailed facets, using diamond dust and lubricant on a lap, which is a disk with abrasives on its surface. Lap use proceeds in stages, first with a coarse lap, to remove excess material, then on to finer laps, to remove scratches, and finally a polishing lap for finishing.
Cutting by laser and computer automation has also improved the consistency in symmetry and the cuts available to diamond cutters.
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References
Resources
- Photo Credit Cleavage plane of diamond (credit: Miriame Bass Jewelry)
