What Rocks Can Be Tumbled: Mohs Hardness Guide

Fluorite rates 4 on the Mohs scale. Quartz rates 7. The numerical difference appears minor—three points on a ten-point scale. In practice, quartz requires four times longer to shape than fluorite using identical tumbler settings and grit. The Mohs scale isn't linear. It's exponential. Corundum at 9 measures twice as hard as topaz at 8, but diamond at 10 rates four times harder than corundum. This exponential progression determines everything about rock tumbling success.

Friedrich Mohs developed the hardness scale in 1812 as a field identification tool. The scale operates on a simple principle: harder materials scratch softer ones. A material that scratches apatite (5) but gets scratched by feldspar (6) rates somewhere between 5 and 6. The scale measures scratch resistance specifically, not overall toughness or brittleness. Diamond rates 10 for scratch resistance but shatters easily on impact. Jade rates only 6.5 to 7 but resists fracture through exceptional toughness.

The Optimal Range for Tumbling

Materials rating 5 to 7 on the Mohs scale tumble successfully in standard rotary tumblers using silicon carbide grit. This range includes most commonly tumbled stones: agate, jasper, petrified wood, quartz varieties, chalcedony, and carnelian. These materials withstand the mechanical action of tumbling while responding appropriately to grit abrasion.

The standard four-week tumbling cycle (one week per stage) was developed specifically for materials rating 7 on the Mohs scale. Agate, jasper, petrified wood, and quartz all cluster around this hardness. They shape predictably under 60/90 coarse grit, smooth effectively with 150/220 medium grit, refine properly using 500 grit, and polish cleanly with aluminum oxide.

Materials rating 6 to 6.5 process faster than the standard cycle. Feldspars including amazonite, labradorite, moonstone, and sunstone all rate approximately 6 on the Mohs scale. These stones complete coarse grinding in 5 to 6 days rather than 7. Tumbling feldspars for the full standard duration removes excessive material, creating smaller finished stones than necessary. Operators working with feldspars often reduce each stage by 1 to 1.5 days, completing the full cycle in approximately 20 to 24 days rather than 28.

Materials rating 5 to 5.5 process even faster. Obsidian (volcanic glass) rates 5 to 5.5. Sodalite rates approximately 5.5 to 6. Turquoise typically measures 5 to 6. These softer materials require careful monitoring during coarse grinding to prevent over-processing. Running obsidian through a full 7-day coarse cycle reduces large pieces to gravel. Most operators limit coarse grinding to 3 to 5 days for these materials, with proportional reductions in subsequent stages.

The guideline approximates to reducing tumbling duration by roughly 1.5 days per Mohs point below 7. A material rating 6 runs approximately 5.5 days per stage rather than 7. A material rating 5 runs approximately 3.5 to 4 days per stage. This represents an average—actual results vary based on specific material characteristics beyond simple hardness ratings.

Materials Below 5: Special Handling Required

Stones softer than 5 on the Mohs scale present significant challenges. Fluorite rates 4. Calcite and most marbles rate 3. Gypsum rates 2. These materials abrade so rapidly that standard silicon carbide grit grinding reduces them to powder before achieving proper shaping.

Some operators successfully tumble fluorite and calcite using modified techniques. The modifications include eliminating coarse grit entirely, starting directly with medium grit for minimal material removal. Reducing tumbling time to 2 to 3 days per stage rather than the standard week. Using ceramic media filler to cushion the soft stones against impacts. Running smaller batch sizes to reduce stone-on-stone contact. Some sources recommend using corn cob media for the final polish rather than aluminum oxide to further reduce abrasion.

These techniques work sometimes but produce inconsistent results. The fundamental problem remains that materials softer than 5 lack the structural integrity to withstand the mechanical stresses of tumbling. Even with careful handling, fractures, chips, and excessive material loss occur frequently. Most experienced operators avoid tumbling materials softer than 5, considering the time and electricity investment unlikely to produce quality results.

Materials Above 7: Extended Processing

Harder materials require extended tumbling durations. Beryl (emerald, aquamarine, morganite) rates 7.5 to 8. Topaz rates 8. Corundum (ruby, sapphire) rates 9. These materials resist grit abrasion more effectively than standard quartz-family stones.

Beryl typically requires 10 to 14 days in coarse grit rather than the standard 7 days. The material's hardness means slower grinding rates—after 7 days, beryl pieces still show angular edges and rough surfaces that would have disappeared on quartz. Extended coarse grinding compensates for this resistance. Subsequent stages also run longer, perhaps 10 days for medium and fine grit rather than the standard 7.

Topaz at 8 on the Mohs scale requires even longer processing. Coarse grinding often extends to 14 to 21 days. The economic calculation becomes questionable at this point—running a tumbler for three weeks to complete the coarse stage alone consumes substantial electricity while producing minimal finished material weight. Most operators working with topaz use it as accent material mixed with harder standard stones rather than processing it in dedicated batches.

Corundum at 9 approaches the practical limits of hobby tumbling. Silicon carbide grit rates 9+ on the Mohs scale, providing barely enough hardness differential to cut corundum effectively. Processing times extend to months rather than weeks. Some operators report running corundum through 8 to 12 weeks of coarse grinding to achieve acceptable shaping. At this level, vibratory tumbling becomes more practical than rotary tumbling—the faster cutting action of vibratory machines compensates somewhat for the material hardness.

Diamond at 10 cannot be tumbled using standard silicon carbide grit. The material's hardness exceeds the grit hardness, preventing effective abrasion. Diamond polishing requires diamond powder as the abrasive medium—an expensive proposition that moves the process beyond hobby tumbling into professional lapidary work.

The Non-Linear Nature of Hardness

The exponential progression of the Mohs scale creates counterintuitive processing realities. The jump from 7 to 8 represents roughly twice the absolute hardness increase as the jump from 6 to 7. The jump from 9 to 10 (corundum to diamond) represents four times the absolute hardness increase as the jump from 8 to 9.

This exponential nature explains why materials rating 8 or above become problematic in hobby tumblers. The hardness increase from 7 to 8 doesn't add 14% more difficulty—it potentially doubles the processing time required. The increase from 8 to 9 doesn't add another 14%—it potentially doubles again.

A sclerometer measures absolute hardness rather than relative scratch resistance. Sclerometer measurements reveal the scale's non-linearity clearly. Talc at 1 measures approximately 1 on absolute hardness. Calcite at 3 measures approximately 9 on absolute hardness—nine times harder than talc despite being only three points higher on the Mohs scale. Quartz at 7 measures approximately 100 on absolute hardness. Diamond at 10 measures approximately 1,500 on absolute hardness—fifteen times harder than quartz despite being only three points higher.

Mixing Hardness Levels: What Actually Happens

Tumbling stones of mixed hardness in a single barrel creates predictable problems. The softer stones abrade faster than harder stones. By the time harder stones achieve proper rounding and smoothing, softer stones have ground down to tiny pieces or powder.

Consider a barrel containing equal weights of agate (7) and obsidian (5.5). After the standard 7-day coarse cycle, the agate shows proper rounding and surface preparation for medium grit. The obsidian has ground down to perhaps 40% of its original size, with excessive material loss creating small, over-processed pieces. Running the batch for only 4 days to protect the obsidian leaves the agate insufficiently processed, with remaining angular edges and rough surfaces that cause problems in subsequent stages.

The economic impact multiplies when working with expensive or rare materials. Processing a pound of obsidian mixed with agate results in perhaps 6 to 7 ounces of finished obsidian due to over-grinding, compared to potentially 10 to 12 ounces if processed separately with appropriate shortened cycles. The material loss translates directly to wasted investment in rough stone.

Separating materials by hardness solves these problems. Running agate and jasper together works well since both rate approximately 7. Running obsidian separately with reduced cycle times maximizes finished stone weight. The separation requires slightly more planning and potentially more barrel capacity, but dramatically improves results.

Hardness Variations Within Stone Types

Individual stones within the same general category often show hardness variations. Agate typically rates 6.5 to 7 depending on composition. Pure chalcedony content pushes toward the softer end. Higher quartz content moves toward the harder end. Mexican lace agate, with its complex banding and inclusions, often tumbles slower than cleaner Brazilian agate despite both theoretically rating 7.

The banding in agate contributes to this variation. Different mineral impurities accumulate in various bands. Clay minerals soften a band. Iron oxides and additional silica harden a band. An individual agate specimen might have hardness variations between 6.5 and 7+ across its different bands. These variations create uneven grinding rates—harder bands remain slightly raised while softer bands grind faster, creating subtle surface texture.

Quartz varieties show similar variations. Clear rock crystal, smoky quartz, amethyst, and citrine all theoretically rate 7 on the Mohs scale as macrocrystalline quartz. In practice, specimens with different impurity content and crystal structures tumble at slightly different rates. Clear quartz crystals often show cleaner, faster processing than heavily included smoky quartz or amethyst specimens containing iron or manganese inclusions.

Jasper presents even more variation since "jasper" describes an entire family of opaque chalcedony rather than a single mineral. Red jasper, green jasper, picture jasper, ocean jasper, and dozens of other varieties all rate nominally at 7 but contain different impurity content. Some jasper types complete processing in the standard 28-day cycle. Others require 5 to 6 weeks to achieve comparable results.

Structural Factors Beyond Hardness

Hardness measures scratch resistance but doesn't account for other structural characteristics affecting tumbling success. Toughness describes resistance to fracture and breakage. Cleavage describes tendency to split along specific crystal planes. Porosity affects how stones interact with water and grit during processing.

Jade demonstrates the distinction between hardness and toughness. Jadeite and nephrite (the two jade types) rate only 6.5 to 7 on the Mohs scale—similar to quartz. Yet jade resists fracture and breakage far more effectively than quartz. The interlocking fibrous crystal structure distributes stress rather than concentrating it at weak points. Jade tumbles successfully despite relatively moderate hardness because its toughness prevents breakage during the mechanical action of tumbling.

Quartz crystals show the opposite pattern. At 7 on the Mohs scale, quartz matches jade's hardness. But quartz cleaves along specific crystal planes. Large quartz crystals tumbled in rotary tumblers often fracture into smaller pieces during coarse grinding. The angular impacts between stones concentrate stress at cleavage planes, creating fractures. Operators working with large quartz crystals often add substantial ceramic media filler to cushion impacts and reduce fracture risk.

Feldspar exhibits even stronger cleavage than quartz. Most feldspars rate 6 to 6.5 on the Mohs scale and show good tumbling behavior when processed carefully. But feldspars cleave along two planes at nearly 90-degree angles. Specimens with visible cleavage planes often fracture during tumbling despite appropriate hardness. Selecting rough feldspars without obvious cleavage planes improves success rates.

Porosity creates different problems. Turquoise ranges from 5 to 6 on the Mohs scale—borderline acceptable for tumbling. The hardness isn't the primary issue. Many turquoise specimens show high porosity, absorbing water and grit during processing. The absorbed material weakens the stone structure, increasing fracture risk. Polishing porous turquoise often results in dull, chalky surfaces rather than glossy finishes because the porous structure scatters light. Stabilized turquoise (turquoise impregnated with resin to fill pores) tumbles more successfully than natural porous material.

Silicon Carbide Grit Hardness and Limitations

Standard silicon carbide tumbling grit rates 9+ on the Mohs scale. This hardness allows it to abrade most commonly tumbled materials effectively. Stones rating 7 or below show significant hardness differential from the grit—the grit easily scratches and abrades the stone surface. Stones rating 8 show reduced hardness differential—the grit still works but cuts slower. Stones rating 9 match the grit hardness approximately—cutting becomes extremely slow.

The hardness match explains why corundum (ruby and sapphire at 9) requires months of processing. Silicon carbide particles and corundum surfaces rate nearly identical hardness. The grit doesn't scratch efficiently. Instead, processing relies on mechanical grinding action—grit particles and stone surfaces slowly wearing each other down through friction and impacts. The process works but proceeds at glacial pace compared to standard quartz tumbling.

Some operators use aluminum oxide grit for extended tumbling of harder materials. Aluminum oxide rates 9 on the Mohs scale—similar to silicon carbide but with different crystal structure and fracture characteristics. The performance differences remain subtle. Both grits struggle with corundum and both work acceptably with materials rating 8 or below.

Boron carbide grit rates approximately 9.5 on the Mohs scale, providing barely enough hardness differential to process corundum more effectively than standard grits. The cost becomes prohibitive—boron carbide sells for ten to twenty times the price of silicon carbide per pound. Few hobby operators use boron carbide except for specific high-value corundum pieces where the time and cost investment makes economic sense.

Diamond powder at 10 on the Mohs scale represents the only practical abrasive for diamond polishing. The cost makes it impractical for hobby tumbling. Commercial lapidary operations use diamond powder for specific high-value work but even there, usage remains limited to final polishing stages rather than bulk material removal.

Practical Testing Methods

Determining stone hardness before tumbling prevents wasted time processing inappropriate materials. The scratch test provides basic field identification. A fingernail rates approximately 2.5 on the Mohs scale. Window glass rates approximately 5.5. A steel knife blade rates approximately 6.5 to 7. Quartz rates 7.

Testing proceeds systematically: Can your fingernail scratch the stone? If yes, the stone rates below 2.5—too soft for tumbling. Can the stone scratch window glass? If no, the stone rates below 5.5—marginal for tumbling at best. Can the stone scratch a steel knife blade? If yes, the stone rates above 6.5—suitable for standard tumbling.

These tests provide approximate results only. A stone that barely scratches glass might rate 5.5 or might rate 6. The distinction matters for processing time calculations but the test cannot determine it precisely. Mohs hardness test kits provide more accurate determination. These kits contain reference minerals or hardness picks representing each point on the scale. Testing a stone against the reference materials determines its hardness within a half-point or better.

Most operators don't test every stone individually. Experience develops pattern recognition—agate and jasper feel a certain way, have particular visual characteristics, and display specific fracture patterns. After processing a few batches, identifying suitable tumbling rough becomes intuitive. The testing remains valuable for unknown materials or when purchasing rough from unfamiliar sources.

Common Stones by Hardness Range

Mohs 9: Corundum Family

• Ruby, sapphire
• Requires 8+ weeks coarse grinding
• Generally impractical for hobby tumbling

Mohs 8: Beryl and Topaz

• Emerald, aquamarine, morganite (beryl family)
• Blue topaz, imperial topaz
• Requires 10-21 days coarse grinding
• Extended processing for all stages

Mohs 7-7.5: Standard Tumbling Range

• Quartz varieties: amethyst, citrine, smoky quartz, rose quartz
• Agate, jasper, carnelian, chalcedony
• Petrified wood
• Tiger's eye
• Standard 7-day cycles work well

Mohs 6-6.5: Fast Processing

• Feldspar family: moonstone, sunstone, amazonite, labradorite
• Unakite
• Reduce cycle times by 1-2 days per stage

Mohs 5-5.5: Careful Processing

• Obsidian (volcanic glass)
• Turquoise
• Sodalite
• Reduce cycle times by 3-4 days per stage
• Monitor closely for over-processing

Mohs 4: Marginal

• Fluorite
• Requires specialized techniques
• Inconsistent results common

Mohs 3 and Below: Not Recommended

• Calcite, marble
• Gypsum, alabaster
• Generally too soft for successful tumbling

When Hardness Guidelines Fail

Certain materials produce poor results despite falling within the 5 to 7 hardness range. Lapis lazuli rates 5 to 5.5—nominally suitable for tumbling. In practice, lapis presents challenges. The material consists of multiple minerals (lazurite, calcite, pyrite) with different hardness levels. The calcite inclusions (Mohs 3) grind away faster than the lazurite matrix (Mohs 5-5.5), creating uneven surfaces. The pyrite inclusions (Mohs 6-6.5) resist grinding, remaining as raised areas. The result often shows poor polish quality despite careful processing.

Malachite rates 3.5 to 4 on the Mohs scale—below the practical tumbling range. Some operators report acceptable results using heavily modified techniques (very short processing times, corn cob polishing media, minimal material quantity). Most attempts produce disappointing outcomes with excessive material loss and poor surface quality.

Opal presents different problems. Hardness ranges from 5 to 6.5 depending on water content—nominally acceptable for tumbling. The high water content creates issues. Opal can lose water during extended tumbling, developing cracks and crazing. The material's amorphous structure (non-crystalline) means it lacks the organized atomic structure that produces clean polishing in crystalline materials. Tumbled opals often show dull, waxy surfaces rather than the brilliant polish achievable on quartz-family stones.

Amber rates only 2 to 2.5 on the Mohs scale—far too soft for tumbling. The organic material (fossilized tree resin) lacks the crystalline structure of mineral stones. Any attempt to tumble amber produces immediate, catastrophic material loss.

Economics of Hardness-Based Processing

Material hardness directly impacts processing economics. A three-pound load of agate running the standard 28-day cycle costs approximately $3-5 in consumable grit plus electricity. The same tumbler running beryl for 70 days (extended cycles for harder material) costs $8-12 in consumables plus substantially more electricity. The time factor multiplies—the tumbler remains occupied for 2.5 months processing beryl versus one month for agate, reducing overall throughput.

Commercial operations factor these economics carefully. Processing standard agate and jasper maximizes profit per tumbler-hour. Processing harder materials makes sense only when the finished stone value justifies extended processing time and increased consumable costs. A load of Brazilian agate rough costing $30 produces perhaps $200-300 worth of finished tumbled stones after 28 days. A load of aquamarine rough costing $200 might produce $500-600 worth of finished stones but requires 70 days—the return on investment drops significantly.

Hobby operators face similar calculations. Time and tumbler capacity represent scarce resources. Dedicating a tumbler to 10-week corundum processing means that tumbler cannot process three standard batches of agate in the same period. The opportunity cost accumulates. Most hobby operators focus on materials rating 6 to 7.5, accepting that harder materials remain outside practical reach.

Related: Rock Tumbling Grit Progression Explained, The Best Rock Tumbler, Stone Identification for Lapidary