Rock Tumbling Grit Log

Rock tumbling grit is an abrasive powder made from silicon carbide or aluminum oxide. It works by gradually wearing down rough rock surfaces inside a rotating barrel. As the barrel turns, the grit particles grind against the rocks, smoothing edges and shaping them. The process uses progressively finer grits across multiple stages—starting with coarse 60/90 grit for heavy shaping, moving to medium 120/220 for smoothing, then pre-polish 500 AO, and finally polish grade for a glossy shine. Each stage typically runs 5–14 days depending on rock hardness and desired results.

As a general rule, use about 1 tablespoon of grit per pound of rock for Stage 1 (coarse 60/90). For Stage 2, use about 0.75 tbsp per pound. For Stages 3 and 4, use about 0.5 tbsp per pound. Example: A 3 lb barrel typically needs ~3 tbsp coarse grit, ~2 tbsp medium grit, and ~1.5 tbsp pre-polish/polish. Our calculator above automatically adjusts based on your selected barrel size. Using too much grit is wasteful; using too little slows progress. Always follow your tumbler manufacturer's guidelines for best results.

Typical durations: Stage 1 (Coarse): 7–14 days—longer for harder rocks like agate or jasper. Stage 2 (Medium): 5–7 days. Stage 3 (Pre-Polish): 5–7 days. Stage 4 (Polish): 5–7 days. These are guidelines; actual times depend on rock hardness (Mohs scale), barrel load, and desired finish quality. Always clean rocks and barrel thoroughly between stages to avoid contamination. Check progress every few days, especially in Stage 1—you can always run longer if edges aren't smooth enough.

Generally, no—do not reuse grit between stages. Grit breaks down and becomes finer as it tumbles. Reusing spent coarse grit means it won't cut effectively. However, some experienced tumblers reuse Stage 1 slurry for a second coarse run if it still has abrasive texture. Always use fresh grit for Stages 2–4 to avoid cross-contamination. A single grain of coarse grit in your polish stage can ruin an entire batch's shine. Cleanliness between stages is critical—scrub rocks, barrel, and lid thoroughly with a brush and soapy water.

Silicon Carbide (SiC) is harder and sharper—ideal for Stages 1 and 2 where aggressive material removal is needed. It fractures into sharp-edged particles, making it excellent for shaping rough rock. Aluminum Oxide (AO) is slightly softer but produces a finer, more consistent scratch pattern—perfect for pre-polish (Stage 3) and final polish (Stage 4). Some polishes use Cerium Oxide (especially for quartz and glassy materials) or Tin Oxide for a high-gloss finish. Match your grit type to the stage for optimal results.

Beginners should start with rocks of similar hardness (Mohs 6–7) for even tumbling. Great choices include: Agate, Jasper, Quartz, Amethyst, Petrified Wood, and Aventurine. Avoid mixing hard rocks with soft ones (like calcite or fluorite), as softer rocks will wear away quickly. Pre-shaped or beach-tumbled rocks speed up results. Many starter kits include a mix of ready-to-tumble rough. Always load your barrel 2/3 to 3/4 full with a mix of rock sizes for efficient grinding action.

Common causes: (1) Contamination—grit from a previous stage trapped in cracks or barrel crevices. Clean thoroughly between stages. (2) Rushing stages—if Stage 1 didn't smooth edges enough, later stages can't compensate. (3) Overloading or underloading the barrel. (4) Using too little polish. (5) Hard water deposits—try a burnishing stage with borax or ivory soap for 1–2 hours after polishing. (6) Rock porosity—some rocks simply won't take a high shine. Ensure each stage runs its full recommended duration before moving on.

Yes—always add water to create a slurry that carries the grit evenly across all rock surfaces. Fill the barrel with water until it just covers the rocks (or reaches the bottom of the top rock layer). Too much water reduces grinding efficiency; too little creates a thick paste that cushions rocks and slows progress. The ideal consistency is a thin, muddy slurry. Check after 24 hours—if it's too thick, add a small amount of water. Never run a tumbler barrel dry with grit.