120 grit silicon carbide is a medium-grade abrasive powder used primarily as the second stage in rotary rock tumbling, bridging the transition between the aggressive shaping of coarse grit and the refinement of finer pre-polish stages. With an average particle size of approximately 120 micrometres, it removes the deeper scratches and surface pitting left by 60 or 80 grit material while continuing to reduce surface irregularities across the entire batch of stones.

Material Properties

Silicon carbide (SiC), sold under trade names such as Carborundum, is a synthetic abrasive first produced industrially by the Acheson process in the 1890s. It registers 9 to 9.5 on the Mohs scale — harder than virtually all ornamental lapidary materials, including quartz (Mohs 7), chalcedony, jasper, agate, and most semi-precious stones. A defining characteristic relevant to its abrasive action is its friability: under tumbling stress, silicon carbide grains fracture along crystallographic planes, continually exposing fresh, sharp cutting edges. This self-sharpening behaviour sustains consistent cutting action throughout a week-long tumbling cycle, unlike some abrasives that glaze over with use.

Role in the Tumbling Sequence

Rotary tumbling conventionally proceeds through a graduated sequence of grits, each stage removing the damage introduced by the previous one. A typical four-stage programme runs as follows:

• Stage 1 — Coarse grind: 60 or 80 grit silicon carbide, shaping rough stones and removing major surface fractures.
• Stage 2 — Medium grind: 120 grit silicon carbide, smoothing coarse-grit scratches and refining overall form.
• Stage 3 — Fine grind or pre-polish: 220 or 400 grit silicon carbide, or aluminium oxide, preparing a near-smooth surface.
• Stage 4 — Polish: Cerium oxide, tin oxide, or aluminium oxide polish, producing the final lustre.

Skipping the 120-grit stage and proceeding directly from coarse to fine grit is a common beginner error; the finer abrasive cannot efficiently remove the deeper scratches left by 60 or 80 grit within a reasonable cycle time, and the resulting polish will show residual surface defects.

Practical Use

The standard loading for a rotary barrel at the 120-grit stage combines the stone charge (typically filling the barrel roughly two-thirds full), a measured quantity of 120 grit silicon carbide powder (commonly one to two tablespoons per pound of stone, though barrel manufacturers publish specific ratios), sufficient water to produce a slurry of medium consistency, and plastic pellets or ceramic media to cushion harder or more fragile specimens and fill void space. The barrel is then run continuously for approximately one week. At the conclusion of the cycle, the slurry — which contains spent abrasive, fine stone particles, and water — must be disposed of carefully; it should never be rinsed down household drains, as silicon carbide slurry can set like concrete in pipework.

Thorough cleaning of both the stones and the barrel between stages is essential. Even a small quantity of 120 grit carried forward into a finer stage will introduce scratches that the finer abrasive cannot remove, effectively negating the subsequent stage's work.

Suitability by Material

120 grit silicon carbide is appropriate for the majority of hard ornamental materials tumbled by hobbyists and small-scale lapidaries: agate, jasper, petrified wood, obsidian, aventurine, and similar silicate minerals with a Mohs hardness of 6 or above. Softer materials — fluorite, calcite, rhodonite, or malachite — may require gentler abrasive sequences or vibratory rather than rotary tumbling, as the mechanical action of a rotary barrel combined with medium grit can cause excessive chipping or rounding loss in softer stones.

In the Trade

Silicon carbide tumbling grits are sold by lapidary suppliers in quantities ranging from small trial packets to bulk bags of several kilograms, typically as part of a matched grit set. The abrasive is inexpensive relative to other lapidary consumables, and 120 grit is among the most commonly restocked grades, as the medium-grind stage often requires slightly longer run times or repeat cycles when working with particularly rough or fractured rough material.