Microcrystalline polish is a polishing compound based on synthetic microcrystalline diamond particles, typically in the 0.25 to 3 micron range, used in the final stages of lapidary work to bring hard gemstones to a high-lustre surface finish. Where coarser diamond grits handle bulk material removal during cutting and pre-polish, microcrystalline diamond is the medium that closes the surface — leaving the polished facets that distinguish a competition-grade stone from a commercial cut.

Particle structure and behaviour

Microcrystalline diamond differs from monocrystalline diamond grit in its internal architecture. The particles are aggregates of much smaller crystallites bonded together, producing a self-sharpening behaviour as the aggregate fractures during use. This characteristic gives microcrystalline diamond a more even cutting action and reduces the random scratching that monocrystalline grit can produce on softer or more cleavage-prone materials. Particle distribution in commercial polishes is tightly controlled — a 1-micron polish should contain particles within a narrow band centred on that size, with minimal coarser material that would drag scratches across the finished facet.

Application

The compound is applied as a slurry, paste, or pre-loaded film to the polishing lap — typically leather, felt, ceran, or one of the proprietary composite materials such as BATT or Last-Lap. Lap material choice is closely tied to the stone being polished: leather and felt are forgiving on softer materials, while harder composites are preferred for corundum and diamond. The cutter uses light pressure and frequent inspection to monitor surface progression, switching from coarser to finer microcrystalline grades through a sequence that might run 3 micron, 1 micron, 0.5 micron, and 0.25 micron for the most demanding work.

Where it earns its place

For sapphire, ruby, alexandrite, and diamond, microcrystalline polish is the standard final-stage medium and effectively non-negotiable for serious cutting. For softer materials such as quartz, beryl, or topaz, cerium oxide or aluminium oxide may produce equivalent or superior results at lower cost. The choice between microcrystalline diamond and cerium oxide on these intermediate materials is often a matter of cutter preference and the specific surface finish target.

Further reading

• GIA — Faceting and lapidary process
• International Gem Society — polishing compounds and lap selection
• United States Faceters Guild — competition cutting standards