Rock Tumbling Grit Progression: The Four-Stage Process Explained

Here's what nobody mentions about rock tumbling: you're creating scratches to remove scratches. The entire four-stage grit progression operates on this principle. Stage one uses 60/90 grit silicon carbide—particles roughly 166 to 250 microns across, visible to the naked eye. These particles grind deep scratches into your stones while removing the rough edges. Stage two introduces 150/220 grit at 63 to 100 microns, creating smaller scratches that erase the damage from stage one. The pattern continues through 500 grit at 25 microns, then final polish at 3 microns. Each stage removes the previous stage's work while preparing the surface for what comes next.

The process compresses approximately 10,000 years of natural erosion into four to eight weeks, depending on stone hardness and desired results.

The Standard Four-Stage Progression

Rock tumbling follows a consistent sequence across rotary tumblers. The progression uses silicon carbide for the first three stages, switching to aluminum oxide for final polish. Each stage operates on specific particle size ranges measured in either mesh or microns—two systems that describe the same physical reality from different angles.

Stage One: Coarse Grind (60/90 Grit)

The first stage uses ungraded 60/90 silicon carbide. "Ungraded" indicates a particle size range rather than uniform sizing—particles pass through a 60 mesh screen but not a 90 mesh screen. This translates to particles between 166 and 250 microns across. A 250-micron particle measures roughly 1/4 millimeter, small enough to see but large enough to feel between fingers.

Silicon carbide rates 9+ on the Mohs hardness scale, harder than any stone typically tumbled. Agate, jasper, and quartz all rate around 7 on the Mohs scale. The hardness differential allows the grit to abrade stone surfaces rather than simply bouncing off them.

Standard rotary tumbler practice uses approximately two tablespoons of grit per pound of rock. For a typical three-pound tumbler barrel running at 2/3 capacity, this translates to roughly four tablespoons or two ounces of coarse grit per stage one run. The grit breaks down during tumbling—the sharp angular particles fracture into progressively smaller pieces while maintaining cutting edges. Silicon carbide's crystal structure ensures each fracture creates new sharp points rather than smooth rounded particles.

Stage one typically runs 7 to 14 days depending on stone hardness and desired shaping. Softer materials like calcite (Mohs 3) or fluorite (Mohs 4) shape in 7 days. Harder materials like quartz or jasper often require 10 to 14 days. Some operators run coarse grit for two or three weeks when working with exceptionally angular rough or when pursuing perfectly rounded results.

The 60/90 grit size represents optimal efficiency for most tumbling scenarios. Experiments with coarser grits like 46/70 or even 20 mesh show counterintuitive results—fewer particles in the barrel means fewer contact points between stones, resulting in less material removal despite larger individual particle size. A test comparing ceramic pellet weight loss across different grit sizes demonstrated that 60/90 grit removed more material in two weeks than either 46/70 or 20 mesh, despite being finer than both.

Stage one produces the most significant physical transformation. Rocks entering the barrel with sharp edges, fracture planes, and irregular surfaces emerge rounded and smooth to touch but covered in visible scratches. The gray mud coating everything at stage one completion consists of spent grit particles (now ground to powder) mixed with stone dust removed during the grinding process.

Stage Two: Medium Grind (150/220 Grit)

After thoroughly washing stage one mud from rocks and barrel, stage two begins with 150/220 silicon carbide. These particles range from 63 to 100 microns—roughly the thickness of a human hair. The particle size reduction from stage one to stage two represents approximately a 60% decrease in diameter.

The same two-tablespoon-per-pound ratio applies. For three pounds of rock plus ceramic media filler, use four tablespoons of medium grit. The barrel should remain 2/3 full even after stage one material loss. Rocks typically lose 20-30% of their mass during coarse grind, requiring ceramic media addition to maintain proper barrel capacity.

Stage two runs 7 to 10 days for most materials. The grinding action continues removing material but at a slower rate than stage one. The primary objective shifts from shaping to surface preparation—removing the deep scratches created by coarse grit while introducing finer scratches that stage three can address.

The distinction between stages becomes evident when examining rocks between steps. Stage one rocks show obvious scratch patterns under direct light. Stage two rocks appear smooth to touch and show reduced scratch visibility, though close inspection reveals the surface remains far from polished.

Contamination represents the critical failure point at stage two. Even a few particles of 60/90 grit remaining in the barrel will create deep scratches that stage two cannot remove. These scratches persist through all subsequent stages, ruining the final polish. Thorough barrel cleaning between stages matters more than any other factor in achieving quality results.

Stage Three: Fine Grind (500 Grit)

The pre-polish stage uses 500F silicon carbide—particles around 25 microns in diameter. The "F" designation indicates "and finer," meaning the grit includes particles at 500 mesh plus smaller sizes down to approximately 15 microns. This particle size approaches the limit of human vision—individual grains become difficult to distinguish without magnification.

Tumbling time decreases to 5-7 days for most materials. The reduced duration reflects both the finer particle size and the minimal material removal required. Stage three removes the 150/220 scratches while creating micro-scratches too small for casual observation. Rocks at this stage show a subtle sheen when wet but appear dull when dry—the surface approaches polish quality but lacks the final luster.

Some operators add a 1000 grit step between stage three and final polish. The additional stage uses approximately the same duration (5-7 days) and follows the same contamination prevention protocols. The 1000 grit measures roughly 13 microns and provides incremental improvement on certain materials, particularly transparent or translucent stones where subtle surface imperfections remain visible.

The diminishing material removal across stages follows a clear pattern. Coarse grit removes perhaps 5-10mm from stone diameter over two weeks. Medium grit removes 1-2mm. Fine grit removes less than 1mm. The progression moves from aggressive shaping through progressive surface refinement, with each stage removing approximately an order of magnitude less material than the previous.

Stage Four: Polish

The final stage switches from silicon carbide to aluminum oxide compounds. The most common rotary tumbler polish uses TXP aluminum oxide centered on 3-micron particle size. Alternative polishes include cerium oxide (particularly effective on quartz family materials), tin oxide for metal/stone combinations, or ultra-fine aluminum oxide formulations like #61 Rapid Polish at 0.3 microns for vibratory tumblers.

Polish stage runs 7 to 14 days in rotary tumblers. The extended duration compared to fine grit reflects the minimal cutting action—polish works primarily through burnishing rather than abrasion. The aluminum oxide particles smooth the micro-scratches from stage three while creating a reflective surface through mechanical action rather than material removal.

The quantity drops to approximately one tablespoon per pound of rock—half the amount used in previous stages. The reduced volume reflects both the finer particle size (more particles per tablespoon) and the minimal material removal objective.

Rocks emerging from polish stage should display mirror-like reflectance when dry. Light should reflect cleanly off the surface rather than scattering. Translucent materials like agate become noticeably more transparent as surface irregularities disappear. The transformation from rough to polished represents a surface quality change of roughly three orders of magnitude—from millimeter-scale irregularities down to micron-scale smoothness.

The Economics of Grit Consumption

Silicon carbide sells for approximately $4.80 to $10 per pound depending on quantity and vendor. A standard one-pound grit kit containing coarse, medium, and fine silicon carbide plus aluminum oxide polish typically costs $15-25. This kit processes roughly four three-pound tumbler loads through all stages.

Breaking down the per-load economics: processing three pounds of rough stone through completion consumes approximately 4 ounces coarse grit, 4 ounces medium grit, 4 ounces fine grit, and 2 ounces polish. At retail pricing, materials cost approximately $3-5 per pound of finished stone before accounting for electricity or rough stone acquisition costs.

Larger operations benefit from bulk purchasing. Twenty-five pound boxes of individual grit sizes reduce per-pound costs to $2-4. The calculation changes substantially at scale—a hobbiest running one three-pound barrel weekly spends roughly $200 annually on consumables, while a commercial operation running multiple large tumblers might spend $2,000-5,000 yearly but achieve per-stone costs under $1.50.

The grit breaks down during use through a combination of particle fracture and size reduction. After 7 days of tumbling, 60/90 grit has largely worn to particles small enough that they stop providing effective cutting. Running the slurry through fingers at week's end reveals no grittiness—the particles have ground themselves into mud. This self-limiting behavior prevents over-grinding while ensuring consistent results across the standard weekly cycle.

Vibratory tumblers consume significantly less grit than rotary tumblers—approximately one-half tablespoon per pound rather than two tablespoons. The reduced consumption reflects the different tumbling action. Rotary tumblers require grit quantity sufficient to coat all rocks as they cascade through the barrel. Vibratory tumblers use rapid shaking motion that keeps grit particles constantly engaged with stone surfaces, allowing less material to achieve similar results.

Time Variables and Their Effects

The standard seven-day cycle per stage represents a practical compromise rather than an absolute requirement. Extending coarse grit from 7 to 14 days produces noticeably rounder stones with smoother surfaces, particularly beneficial when starting with highly angular rough. Running coarse grit for three or four weeks creates near-spherical results from jagged fragments, though diminishing returns set in after week two.

Conversely, cutting stage times below the standard duration risks incomplete surface preparation. Running stage one for only 5 days might leave insufficient rounding, creating problems in subsequent stages. The flat spots or sharp edges remaining after abbreviated coarse grinding persist through all later stages since medium and fine grit lack the aggressive cutting needed for major shaping.

Stone hardness affects optimal duration more than any other variable. Quartz and jasper at Mohs 7 require the full standard duration or longer. Obsidian at Mohs 5.5 often completes satisfactorily in 5-7 days per stage rather than the full week. Feldspars at Mohs 6-6.5 fall between these extremes. Mixing stones of significantly different hardness in a single barrel creates problems—the softer materials over-process while harder materials under-process.

Temperature influences grit breakdown rate. Summer heat accelerates the chemical processes alongside mechanical grinding, occasionally requiring slightly shorter cycles. Winter cold slows reactions, sometimes warranting an extra day or two per stage. The effect remains subtle enough that most operators ignore it, but commercial operations running multiple barrels year-round sometimes adjust schedules seasonally.

What Contamination Actually Does

Cross-contamination between stages represents the single most common cause of poor results. A few grains of 60/90 coarse grit remaining in the barrel when starting stage two will create deep scratches as they tumble alongside the stones. The 150/220 medium grit cannot remove scratches created by particles three times its size. Those scratches persist through fine grit and polish, appearing as visible lines across the otherwise smooth surface.

The contamination works both directions. Fine grit contaminating a coarse stage creates no problems—the fine particles simply join the natural breakdown products. But coarse particles in a fine stage ruin the batch. This asymmetry explains why some operators maintain separate barrels for each stage, eliminating cleaning as a variable.

Proper cleaning between stages involves thorough barrel rinsing, rock washing in a mesh strainer, and verification that no grit remains. The spent slurry should not go down drains—silicon carbide settles in pipes and acts like cement when dry. Most operators pour slurry into a plastic bucket, allowing the solids to settle before carefully decanting the water. The settled grit and rock powder can be disposed of in regular trash.

Observable Differences Between Stages

Examining rocks at each stage reveals the progression's physical reality:

After Stage One: Rocks feel smooth to touch but show obvious scratch patterns when held under direct light. The surface appears matte gray or tan depending on stone color. Edges are rounded and sharp points are removed. The shape established at this stage remains essentially unchanged through completion—subsequent stages only refine the surface.

After Stage Two: Scratches become less obvious to casual inspection but remain visible under close examination. The surface begins showing hints of the final color rather than appearing universally gray. Wet rocks show slight sheen. The feel remains smooth but not slippery.

After Stage Three: Scratches become difficult to detect without magnification. Dry rocks show subtle sheen and the true stone color emerges clearly. Wet rocks display obvious gloss. The surface feels notably different—smoother and slightly slippery compared to stage two. Translucent materials begin showing interior features previously obscured by surface irregularities.

After Stage Four: The surface reflects light cleanly rather than scattering it. Dry rocks show mirror-like gloss. Translucent materials display maximum clarity. The surface feels glass-smooth and slightly waxy. Running a fingernail across the surface produces no catch or drag.

The progression from rough to polished represents a journey from millimeter-scale to micron-scale surface quality. Each stage reduces surface irregularities by roughly an order of magnitude. The cumulative effect transforms stones that look like gravel into specimens resembling glass marbles.

Alternative Progressions and Modifications

Some operators use non-standard progressions for specific materials or objectives. A five-stage progression adds 1000 grit between pre-polish and final polish, requiring an additional 5-7 days but producing superior results on transparent materials. A six-stage progression further adds 120 grit between coarse and medium, useful when working with particularly hard materials requiring extended shaping.

Vibratory tumblers typically skip coarse grit entirely, starting with 150/220 medium grit. The rapid vibration action prevents coarse particles from maintaining contact with stone surfaces—they simply bounce around ineffectively. Vibratory operations run 12-24 hours per cycle rather than 7 days, checking progress and recharging with fresh grit until stones reach desired smoothness. Total vibratory time typically spans 1-2 weeks compared to 4-8 weeks for rotary processing.

Some materials require specialized polishes. Jade responds better to chrome oxide than standard aluminum oxide. Lapis lazuli benefits from similar treatment. Softer stones like turquoise (Mohs 5-6) or fluorite (Mohs 4) sometimes require adjusted grit quantities to prevent over-processing—using 1.5 tablespoons per pound rather than 2 tablespoons reduces material removal rate.

The standard progression assumes roughly equal hardness across all stones in a batch. When tumbling mixed hardness materials, some operators use a modified approach: run coarse stage until the hardest materials reach desired shape, remove softer materials that completed earlier, then continue with a barrel containing only similar-hardness stones. This prevents over-processing soft materials while ensuring hard materials complete properly.

The Reality of Grit Breakdown

Silicon carbide particles don't wear smooth like river rocks. The crystal structure ensures each fracture exposes new sharp edges. A 60/90 particle tumbling for several days fractures into progressively smaller angular fragments, maintaining cutting ability even as size decreases. This explains why coarse grit remains effective for 7-10 days rather than becoming dull after hours.

The breakdown follows a predictable pattern. Fresh 60/90 grit measures 166-250 microns. After 24 hours of tumbling, the largest particles have fractured to roughly 150-200 microns. By day three, the size distribution centers around 100-150 microns. By day seven, most particles measure under 100 microns—approaching medium grit territory. At this point the coarse stage effectiveness drops significantly since the grit now overlaps with medium rather than providing the aggressive cut that coarse grit should deliver.

The self-limiting behavior provides built-in process control. Tumbling coarse grit for three weeks doesn't create problems because the grit breaks down to ineffective size before over-processing occurs. This same principle explains why extending medium or fine stages by several days rarely causes issues—the grit wears itself out naturally.

Practical Verification Methods

Testing grit condition requires nothing more than fingers. After tumbling for the planned duration, open the barrel and pour a small amount of slurry into your palm. Rub the slurry between thumb and fingers. Fresh coarse grit feels obviously gritty, like coarse sand. Spent coarse grit after 7-10 days feels smooth, perhaps slightly gritty but mostly mud-like. This tactile test indicates when the stage has run its course.

The same test works for medium and fine stages, though the distinction becomes more subtle. Fresh 150/220 medium grit feels like fine sand. Spent medium grit feels smooth with minimal grittiness. Fresh 500 fine grit feels like powder with slight texture. Spent fine grit feels completely smooth.

Visual inspection provides secondary confirmation. Pour a small amount of fresh grit into a white container alongside a sample of spent grit. The fresh material shows obvious particle size and sharp angular edges under magnification. The spent material appears as fine powder with few visible particles.

When to Extend or Reduce Duration

Several indicators suggest extending a stage beyond standard duration. If rocks at stage one completion show insufficient rounding or significant flat spots remain, run additional coarse grit. If stage two rocks still show obvious scratch patterns under direct light, run additional medium grit. If stage three rocks lack the subtle sheen expected at pre-polish completion, run additional fine grit.

Reducing duration below standard rarely makes sense except when processing notably soft materials or when repeating a stage on stones that nearly completed previously. The standard 7-day cycles represent minimum effective duration for most scenarios rather than maximum safe duration.

The exception involves running stages substantially longer than needed. Tumbling coarse grit for a month doesn't harm results but wastes time and electricity since most effective grinding completes within 10-14 days. Similarly, running polish for three weeks provides minimal improvement over two weeks—the diminishing returns kick in quickly as the surface approaches its theoretical maximum smoothness.

Related: Starting Lapidary Work: First Equipment, The Best Rock Tumbler, Lapidary Work Water Requirements