Diamond polishing paste — also widely known in the trade as diamond compound — is an oil-suspended abrasive preparation containing precisely graded synthetic diamond particles. Applied to faceting laps, polishing wheels, or flat laps, it enables the lapidary to progress from coarse stock removal through to a mirror-quality surface finish. Its particular importance lies in the polishing of hard or refractory materials — corundum (sapphire and ruby), chrysoberyl, spinel, and topaz — where conventional oxide polishing powders such as cerium oxide or aluminium oxide lack sufficient hardness to cut efficiently.

Composition and Grading

The active component is synthetic diamond grit, manufactured by high-pressure high-temperature (HPHT) synthesis and subsequently milled and classified to tight particle-size tolerances. The carrier medium is either a petroleum-based oil or a synthetic lubricant; its role is twofold — to suspend the diamond particles in a workable paste and to provide hydrodynamic lubrication at the lap–gemstone interface, preventing heat build-up and minimising the risk of fracture in sensitive stones.

Grades are expressed in microns (µm), with the following sequence representing the most commonly stocked steps in a professional lapidary workshop:

• 60 µm — coarse pre-polish; aggressive stock removal on hard materials
• 15 µm — medium pre-polish; removes scratches left by coarser grits
• 6 µm — fine pre-polish; surface begins to develop translucency
• 3 µm — transition grade; prepares the surface for final polishing
• 1 µm — fine polish; produces high lustre on most gem materials
• 0.5 µm and 0.25 µm — ultra-fine (sub-micron) grades; mirror finish on the hardest species

Sub-micron grades — those below 1 µm — are sometimes marketed separately as sub-micron diamond or diamond slurry, particularly when supplied in a more fluid suspension intended for use on ceramic or tin-alloy laps. The distinction between paste and slurry is largely one of viscosity and intended substrate rather than any fundamental difference in abrasive action.

Application and Lap Compatibility

Diamond paste is typically dispensed from a graduated syringe or small jar and applied sparingly to the lap surface. A small quantity — often no more than a few millimetres extruded from a syringe — is sufficient for a polishing session; overloading the lap with paste causes the particles to agglomerate and can paradoxically produce a coarser scratch pattern. The paste is spread across the lap with a fingertip or a clean cotton swab before the stone is brought into contact.

Lap material is critical to the outcome. Diamond paste performs best on laps that allow the particles to become partially embedded or to roll freely across a consistent surface. Commonly paired substrates include:

• Tin or tin-alloy laps — the traditional choice for final polishing of corundum and spinel; the relatively soft metal allows diamond particles to charge the surface evenly
• Ceramic laps — dense, flat, and dimensionally stable; suited to finer grades and hard species
• Copper laps — used with coarser grades for pre-polishing hard materials
• Leather or felt laps — occasionally used with the finest sub-micron grades for a final brightening pass

Cross-contamination between grades is one of the most common sources of polishing failure. Dedicated laps for each grit size, or rigorous cleaning protocols between grades, are standard practice in any serious workshop.

Use with Hard Gem Species

For gem materials at the upper end of the Mohs scale — corundum (Mohs 9), chrysoberyl (Mohs 8.5), and spinel (Mohs 8) — diamond paste is often the only practical route to a high-quality polish. Oxide powders, which rely on a chemical–mechanical polishing action, are effective on softer or more chemically reactive materials such as quartz or feldspar, but their hardness is insufficient to cut efficiently into corundum or chrysoberyl. Diamond, as the hardest known natural material, cuts all gem species without exception, making diamond paste universally applicable across the full hardness range.

Alexandrite and other chrysoberyl varieties, with their pronounced cleavage resistance but demanding polish requirements, are routinely finished with 1 µm or 0.5 µm diamond paste on a tin lap. Similarly, fine star sapphires and star rubies — polished as cabochons rather than faceted — benefit from a careful progression through diamond paste grades to achieve the smooth, highly reflective dome surface necessary for asterism to display cleanly.

Suppliers and Formats

Diamond polishing paste is produced by a number of specialist lapidary and industrial abrasive suppliers. In the gemstone cutting community, well-regarded sources include Crystalite Corporation and Gearloose Lapidary, both of which supply paste in syringe format with colour-coded grades for easy identification. Industrial diamond compound manufacturers also supply the lapidary trade, and their products are often chemically identical to purpose-branded lapidary versions. Paste is available in water-soluble (water-based carrier) and oil-soluble formulations; the choice depends on the lap material and the lapidary's preferred coolant or lubricant system.