220 grit silicon carbide is a medium-grade abrasive used in rotary and vibratory tumbling as an intermediate grinding stage, positioned between coarse grinding compounds (typically 80 or 120 grit) and the fine pre-polish stages that follow. Its role is precise: to remove the deeper scratches and surface irregularities introduced by coarser abrasives, reducing surface roughness without yet approaching the smoothness required for polishing. In the systematic progression of lapidary tumbling, this stage is often the one that determines whether a finished stone will take a clean, even polish or retain a subtly matte, scratched surface beneath its apparent shine.

Silicon Carbide as an Abrasive

Silicon carbide (SiC), known mineralogically as moissanite in its natural form, is a synthetic compound produced by fusing silica sand and carbon at high temperature — a process developed by Edward Acheson in the early 1890s and still the basis of commercial production. With a Mohs hardness of approximately 9 to 9.5, silicon carbide is harder than the vast majority of lapidary materials, including quartz (Mohs 7), agate, jasper, chalcedony, and most ornamental stones commonly tumbled by hobbyists and small-scale producers. This hardness differential ensures that the abrasive cuts the stone rather than the reverse, and that grit particles fracture progressively during use, continually exposing fresh cutting edges — a property known as friability, which distinguishes silicon carbide from harder but less friable alternatives such as aluminium oxide in certain applications.

The grit designation system used for tumbling abrasives follows the mesh sizing conventions of the abrasives industry: a 220 grit designation indicates that the particles pass through a mesh with approximately 220 openings per linear inch, yielding an average particle size of roughly 53 to 68 micrometres. This places 220 grit in the medium range — substantially finer than the 80 grit used for initial shaping, yet coarse enough to remove the scratch pattern left by 120 grit efficiently.

Role in the Tumbling Sequence

Standard rotary tumbling for hard siliceous materials such as agate, petrified wood, and jasper typically follows a four-stage sequence: coarse grind (80 or 120 grit), medium grind (220 grit), pre-polish (500 or 600 grit, or aluminium oxide), and polish (cerium oxide, tin oxide, or similar). The 220 grit stage serves as the critical refinement step. Stones entering this stage carry visible scratches from coarse grinding; stones leaving it should display a uniformly matte, scratch-free surface under magnification — a condition sometimes called a satin finish in the lapidary trade — that will respond evenly to subsequent polishing compounds.

Tumbling duration at 220 grit typically ranges from one to two weeks in a rotary barrel, depending on the hardness and toughness of the material, the barrel load, the water-to-grit ratio, and the condition of the stones entering the stage. Vibratory tumblers generally achieve the same surface refinement in a shorter period and with less rounding of edges and corners, making them preferable when preserving the natural shape of a rough piece is desirable.

Practical Considerations

Effective use of 220 grit silicon carbide requires attention to several variables:

• Barrel cleanliness: Residual coarse grit carried over from the previous stage will contaminate the 220 grit slurry and introduce deep scratches that defeat the purpose of the finer compound. Thorough washing of both stones and barrel between stages is essential.
• Grit quantity: Standard guidance recommends approximately two level tablespoons of 220 grit per pound of rough material, though this varies by manufacturer recommendation and barrel geometry.
• Water level: The slurry should be thick enough to coat all stone surfaces but not so dilute that abrasive particles are merely suspended rather than in contact with stone surfaces. The barrel is typically filled to about two-thirds capacity with stones, with water added to just below the top of the stone load.
• Material segregation: Mixing stones of significantly different hardness in the same batch risks softer material being over-ground while harder material remains under-processed. Stones of similar hardness and comparable size tumble most evenly together.

In the Trade

Silicon carbide tumbling grits are sold in pre-packaged grit kits by lapidary suppliers and as bulk quantities for higher-volume use. The 220 grit stage is sometimes omitted by hobbyists working with very soft materials or seeking a more rounded, heavily frosted finish, but for hard siliceous stones destined for a bright polish, skipping it generally produces inferior results — the polish stage cannot compensate for a scratch pattern that pre-polish compounds have not fully addressed. Among experienced lapidaries, the quality of the 220 grit stage is widely regarded as the single greatest determinant of final polish quality in rotary tumbling.