A lap, in lapidary practice, is the flat rotating disc on which a gemstone is ground, cut, or polished. The lap is the working surface of a faceting machine or a cabochon grinder, and the choice of lap material, grit, and binder defines the cut quality at every stage from preforming to final polish. A faceter typically uses a sequence of laps over the course of a single stone – a coarse cutting lap to establish the geometry, finer pre-polish laps to remove subsurface damage, and a polish lap charged with diamond, alumina, or cerium oxide to bring the facet to its final reflective surface.

Materials

Lap material is selected by the work the stone requires. Master laps are precision-flat metal substrates – usually steel or aluminium – on which thin removable laps mount. Working laps include sintered diamond on steel, electroplated diamond on copper or steel, copper or tin solid metal laps, ceramic laps, lead-tin alloys (Batt and similar proprietary types), and resin-bonded composite laps. Topaz, beryl, garnet, quartz, and corundum each respond differently to lap material: corundum requires harder laps and slower lap progressions because of its hardness, while quartz polishes well on cerium oxide on a lead-tin lap. Diamond cutters use cast-iron scaifes charged with diamond grit and oil, a specialised case of the same principle.

Sizing and speed

Faceting laps are commonly 6 inches (152 mm) or 8 inches (203 mm) in diameter, mounted on a horizontal spindle running between roughly 200 and 1500 RPM. The cutter selects speed for the lap-and-stone combination: quartz on tin runs slower than corundum on a sintered diamond lap. A drip irrigation system delivers water or a water-glycol coolant to the lap surface to flush swarf and to keep the stone temperature low enough to avoid thermal damage. Cabochon machines use larger laps and wheels, including 6- and 8-inch flat laps and 6-inch and 8-inch grinding wheels.

Charging and dressing

Solid metal laps are charged with abrasive – diamond paste, oxide powder in slurry, or pre-mixed compounds – rather than coming pre-bonded. A lap charger or olive-oil paste applicator distributes the abrasive, and a dressing stick (a wood or composite block) presses charge into the lap surface. Sintered or plated diamond laps come pre-charged at manufacture and are not user-rechargeable; when their cutting surface degrades, they are replaced or, in the case of some plated laps, returned for re-plating. Lap dressers are the tools used to true the surface of a worn lap, removing high spots and restoring flatness.

Care

A faceter's laps live or die by their flatness and cleanliness. Cross-contamination between grits ruins fine work: a single 600-grit particle on a 50,000 polish lap leaves a scratch the cutter must remove with re-polishing. Laps are stored in protective sleeves, dedicated cabinets, or drawers, and most cutters reserve a separate set of polish laps for each major polish material to prevent contamination. The economics of faceting reflect this: a serious cutter's lap inventory is often the single largest line item on the bench beyond the machine itself.