A 600 grit cab belt is a fine-abrasive sanding belt used in the cabochon-grinding sequence, positioned between the medium-grit shaping stages and the final polish. Mounted on an expanding rubber drum or flat lap and used wet, it removes the surface scratches introduced by coarser belts — typically 220 or 320 grit — and leaves a uniformly refined surface ready to accept polishing compounds. The step is critical: any scratch pattern that survives into the polish stage will be amplified rather than erased by the finer abrasives that follow.

Abrasive Composition

600 grit cab belts are manufactured from one of two principal abrasive media. Silicon carbide (SiC) is the more common choice for general lapidary work; it cuts aggressively relative to its grit size and is well suited to softer materials such as opal, turquoise, rhodonite, and most chalcedony varieties. Aluminium oxide (Al₂O₃) belts are harder-wearing and preferred for tougher or harder materials, including jasper and nephrite jade. Both types are bonded to a flexible cloth or polyester backing that conforms to the curved profile of a cabochon without introducing flat spots.

Role in the Grit Progression

Lapidary practice follows a disciplined sequence of increasingly fine abrasives, each stage removing only the scratch pattern left by its predecessor. A typical progression for cabochon work runs through 80 or 100 grit (rough shaping), 220 grit (refining the dome profile), 320 grit (removing coarse scratches), 600 grit (pre-polish smoothing), and then polishing compounds applied on leather, felt, or a dedicated polishing wheel. Skipping the 600 grit stage — moving directly from 320 grit to polish — is a common beginner error that results in a hazy or orange-peel surface rather than a true high lustre.

At 600 grit, the abrasive particles are fine enough that the surface begins to take on a slight sheen under raking light, which serves as a useful visual check: if the stone still appears uniformly matte and scratched at this stage, additional time on the 600 grit belt is warranted before proceeding.

Working Technique

Water is applied continuously — either by a drip feed or by keeping a wet sponge in contact with the drum — to flush swarf, prevent heat build-up, and extend belt life. Heat is particularly damaging to thermally sensitive materials: opal can craze, and some dyed or resin-stabilised stones may delaminate or discolour if run dry. Light, even pressure and consistent rotation of the stone across the belt surface ensure an even scratch pattern and prevent localised flat spots on the dome.

Belt life at the 600 grit stage is shorter than at coarser grits because the finer abrasive particles dull more quickly. A glazed or loaded belt — one whose pores are clogged with swarf — will generate heat rather than cut cleanly and should be replaced. Many lapidaries keep a dressing stick on hand to lightly abrade a glazed belt back to a cutting surface, though replacement is generally preferred for critical pre-polish work.

Materials Particularly Sensitive to This Stage

• Opal: Requires careful wet grinding at all stages; the 600 grit step is especially important because opal's relatively low hardness (5.5–6.5 on the Mohs scale) means coarser scratches can penetrate deeply and prove difficult to polish out.
• Turquoise: Porous and often stabilised; heat and excessive pressure at any grit stage can alter surface colour or compromise resin treatments.
• Chalcedony and agate: These microcrystalline quartz varieties take an excellent polish, but only if the 600 grit stage has produced a truly uniform surface; any residual coarse scratches will show as bright lines in the finished cab.
• Malachite and azurite: Soft and chemically sensitive; a light touch at 600 grit prevents undercutting of the banded structure.

Transition to Polish

After the 600 grit belt, some lapidaries insert an intermediate 1200 grit or 3000 grit stage — available as belts, discs, or flexible laps — before moving to polishing compounds. This extended progression is particularly recommended for hard, high-lustre materials such as chrysoberyl or corundum cabochons, where the final polish demands an exceptionally refined pre-polish surface. For softer materials, the 600 grit belt is often the final abrasive stage before cerium oxide, tin oxide, or diamond paste is applied on a leather or felt wheel to bring the stone to its finished lustre.