Polishing
Polishing
The final stage of gemstone cutting, where facet surfaces are refined to a high lustre
Polishing is the final stage of gemstone cutting, in which the surfaces of facets, cabochon domes, and bead surfaces are refined to a high lustre using progressively finer abrasives on rotating laps, wheels, or in tumbling barrels. The operation removes the scratches left by earlier grinding and shaping stages and brings the surface to the point where light is reflected and refracted cleanly, maximising brilliance and visual lustre. Polishing is the operation that distinguishes a finished gemstone from a roughed-out shape.
How polishing works
The polishing process exploits the same principle as the earlier grinding and lapping stages — abrasive action on the gemstone surface — but at progressively finer grit sizes. Where bruting and blocking use coarse diamond grit to remove material quickly, polishing uses very fine abrasives at the limit of what can produce visible surface improvement. The result is the removal of microscopic surface roughness rather than bulk material, and the operation is correspondingly slower and more delicate.
The choice of abrasive depends on the species being polished. Diamond powder, with its hardness greater than any other gemstone, polishes diamond and the harder coloured species. Cerium oxide, an effective polish for silica and similar species, is the standard choice for quartz and topaz. Aluminium oxide, which is softer and slower-cutting than cerium oxide, polishes a range of coloured stones including beryl, tourmaline, and corundum at appropriate grit. The lap on which the abrasive is mounted also matters: metal laps for harder polishing, ceramic for medium work, wax or wood for softer or more controlled polishing.
Cabochon and bead polishing
Faceted stones are polished facet by facet on a flat lap, with the operator presenting each surface to the lap at the correct angle. Cabochons and beads, with their curved surfaces, require different equipment. Tumbling in barrels with progressively finer abrasive media — silicon carbide grit at the coarse stages, polishing slurries with cerium oxide or aluminium oxide at the fine stages — is the typical approach for production-scale cabochon and bead work, with bamboo chips, walnut shell, or felt media used in the final polishing tumbling stage.
Hand-polishing of cabochons on a felt or leather lap charged with rouge or polishing slurry remains the practice for higher-quality work and for shapes too complex for tumbling. The cutter rotates the cabochon dome against the lap, working the surface progressively to the final lustre.
Polishing in the production sequence
In diamond cutting the polishing stage follows blocking and faceting, with the polisher's work bringing the surfaces from the matte finish left by faceting to the mirror finish required for laboratory grading. The transition between faceting and polishing is the moment in the production sequence when the stone takes on its finished appearance, and observers watching a diamond come off the polishing wheel for the first time see the visual effect they have been waiting for through the previous operations.
For coloured stones the same general sequence applies, although the boundaries between faceting and polishing are sometimes less sharp. A skilled cutter may transition smoothly between fine faceting and initial polishing on the same lap, varying the abrasive grade rather than swapping equipment. The principle is the same: progressively finer surface treatment until the polish grade target is reached.
What good polishing produces
A well-polished facet returns light cleanly through total internal reflection, with no surface scattering to dim the brilliance the cutter has designed into the stone's geometry. A well-polished cabochon dome reflects an even, sharp highlight that moves smoothly as the stone is rotated. A well-polished bead has a uniform lustre across its surface with no dull patches or visible polishing marks. The standard, in each case, is what the species and the intended use will allow: diamond targets the highest lustre any natural material can achieve; opal and turquoise polish to a softer lustre appropriate to their lower hardness.