A 280 grit cab belt is an abrasive belt fitted to a cabochon-grinding machine and designated by its grit number — a standardised measure of abrasive particle size in which higher numbers indicate finer particles. At 280 grit, the belt occupies the middle ground of the cabbing sequence: coarse enough to remove the deeper scratches left by 80 or 120 grit shaping belts, yet fine enough to begin the transition toward a smooth, pre-polish surface. It is one of the most consistently used intermediate stages in lapidary practice.

Position in the Grit Sequence

Cabochon cutting follows a progressive sequence of abrasive stages, each removing the scratch pattern of the previous one. A typical sequence runs 80 grit (rough shaping), 120 or 220 grit (refined shaping), 280 grit (intermediate smoothing), 400 grit, 600 grit, 1200 grit, and finally a polishing compound on a leather, felt, or canvas wheel. Skipping stages — moving directly from 120 to 600 grit, for example — leaves subsurface scratches that polishing compounds cannot fully erase, resulting in a hazy or orange-peel finish rather than a true mirror polish.

The 280 grit stage is particularly important for harder gem materials such as quartz, chalcedony, and feldspar, where the scratch depth left by coarser belts is significant and must be methodically reduced before finer abrasives are applied.

Abrasive Composition

The two abrasive materials most commonly used in 280 grit cab belts are silicon carbide (SiC) and aluminium oxide (Al₂O₃). Silicon carbide, with a Mohs hardness of approximately 9 to 9.5, is aggressive and well suited to hard silicate minerals; it fractures during use, continually exposing fresh cutting edges, which makes it efficient but somewhat shorter-lived. Aluminium oxide, slightly softer at around 9 on the Mohs scale, tends to wear more gradually and is often preferred for materials that benefit from a less aggressive cut, such as softer ornamental stones or organic gem materials like jet or amber. Diamond-impregnated belts at equivalent grit sizes are also available for the hardest gem materials, though they are considerably more expensive and less commonly used at the 280 grit stage.

Practical Use

When working at the 280 grit stage, the lapidary applies moderate, even pressure and keeps the stone moving continuously across the belt to avoid localised flat spots or heat build-up. Water cooling — either via a drip system or a wet sponge positioned against the belt — is standard practice; it prolongs belt life, reduces dust, and prevents thermal shock to heat-sensitive stones such as opal or turquoise. The stone should be inspected under good lighting at regular intervals: the surface should show a uniform, fine scratch pattern with no remaining coarser scratches before the lapidary advances to the next stage.

Belt wear is a practical consideration. A 280 grit belt that has been used extensively will behave more like a 400 grit belt, which can be advantageous as a transitional step but may also mislead a less experienced lapidary into believing the surface is more refined than it actually is. Consistent results depend on using belts in known condition.

Selection and Compatibility

Cab belts are manufactured in standard widths and circumferences to fit specific machine models — common sizes include 6 × 2½ inch and 6 × 89 inch configurations for popular flat-lap and expandable-drum machines. When purchasing 280 grit belts, the lapidary should confirm both the machine's drum or platen dimensions and the abrasive type appropriate for the gem materials being worked. Silicon carbide belts are widely available from lapidary suppliers and represent the standard choice for general cabochon work across quartz-family and silicate gem materials.