90 grit silicon carbide (SiC) is a coarse-to-medium lapidary abrasive with an approximate particle size of 150–180 microns, used principally in the second stage of rotary tumbling and as a grinding medium on flat laps for cabochon work. With a Mohs hardness of 9 to 9.5, silicon carbide is harder than the vast majority of gem materials encountered in lapidary practice, making it an efficient agent for removing the deeper scratches and surface irregularities left by the initial shaping grits — typically 60 or 80 grit — without introducing the gross material loss associated with those coarser stages.

Role in the Tumbling Sequence

Rotary tumbling proceeds through a graduated sequence of abrasive stages, each finer than the last, with the objective of progressively reducing surface relief until a polish can be achieved. The 90 grit stage occupies a critical intermediate position: coarse enough to work efficiently on hardened material such as agate, jasper, or petrified wood, yet fine enough to begin the transition toward smoothing. A typical run at this grit lasts seven to ten days, depending on the hardness of the material, the weight and composition of the barrel load, and the ratio of abrasive to water. Undercutting this stage — moving prematurely to 220 or 400 grit — is among the most common causes of poor final polish, as residual 90-grit scratches are difficult to eliminate at finer stages without significantly extending run times.

Material Compatibility

Silicon carbide abrasives are suited to materials across a broad hardness range. In the context of 90 grit specifically, the abrasive is most commonly applied to quartz-family stones (hardness approximately 7 on the Mohs scale), feldspars, and similarly hard silicate minerals. Softer materials — calcite, fluorite, or certain phosphates — may be abraded too aggressively at this grit and are better introduced at a finer stage or processed in dedicated soft-stone barrels. Mixing materials of substantially different hardness in a single barrel load is inadvisable, as harder stones will abrade softer ones unevenly.

Use on Flat Laps

Beyond rotary tumbling, 90 grit silicon carbide is employed on cast-iron or steel flat laps for the initial grinding of cabochons. In this context it removes saw marks and establishes the dome profile before the lapidary progresses to 220 grit and subsequently to pre-polish wheels. The abrasive is applied as a slurry — powder mixed with water — and fed onto the lap surface during grinding. Silicon carbide laps are generally preferred over diamond laps at this coarse stage for cost reasons, though diamond wheels have become increasingly common in contemporary lapidary studios for their consistency and longevity.

Handling and Safety

Silicon carbide powder is an inhalation hazard in its dry state. Standard lapidary practice requires that the abrasive be kept wet during use and that dry powder be handled with appropriate respiratory protection. Spent slurry from tumbling barrels should be allowed to settle before disposal; silicon carbide is chemically inert but should not be discharged in quantities that may block drainage systems. Barrels, lids, and all equipment should be thoroughly rinsed between grit stages to prevent cross-contamination, which can introduce coarse particles into finer-grit runs and compromise surface quality.

Progression After 90 Grit

Once the 90 grit stage is complete and stones display a uniformly matte surface free of visible pits or deep scratches, the lapidary advances to 220 grit or, in some sequences, directly to 400 grit for smoothing. These finer stages are followed by pre-polish compounds — commonly aluminium oxide or cerium oxide — and a final polish stage. The integrity of the finished surface is directly traceable to the thoroughness of each preceding stage, and 90 grit represents the last point at which significant material removal is both practical and expected.