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.

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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.

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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.

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