Before you choose a grit, it helps to know how hard your stone is. The Mohs scale ranks minerals from 1 (talc) to 10 (diamond), and each one scratches the ones below it. Stones of similar hardness tumble well together, and the hardness tells you how aggressive your grit and schedule should be.
Find your material below. Each group lists the media we recommend to start with. Hardness is approximate: natural stones vary, and cracks, grain and porosity matter as much as the number.
- Hard stones (6.5 to 7.5)
- Medium stones (5.5 to 6.5)
- Softer and brittle stones (5 to 5.5)
- Very soft materials (below 5)
- Metals
- The Mohs reference scale
Hard stones: 6.5 to 7.5
Best for tumbling. Use the standard four-step sequence: 60/90 coarse, 120/220 medium/fine, 500 pre-polish and aluminum oxide polish. Very rough or large pieces can start with 36 or 46/70. Our grit kits cover the full sequence.
| Material | Mohs hardness | Notes |
|---|---|---|
| Agate (including Lake Superior agate) | 6.5 to 7 | The classic tumbling stone. Takes a glassy polish. |
| Jasper | 6.5 to 7 | Polishes well; some grainy pieces stay slightly satin. |
| Chalcedony, carnelian, onyx | 6.5 to 7 | Same family as agate. Standard schedule. |
| Quartz (clear, rose, smoky) | 7 | Watch for internal fractures that can split. |
| Amethyst, citrine | 7 | Quartz varieties. Standard schedule. |
| Tiger’s eye | 7 | Polishes to a bright, silky shine. |
| Aventurine, quartzite | 6.5 to 7 | Granular; may take an extra polish run. |
| Petrified wood | 6.5 to 7 | Check for soft or cracked layers before loading. |
| Bloodstone, chert, flint | 6.5 to 7 | Standard schedule. |
| Garnet | 6.5 to 7.5 | Hard; often small pieces, so add filler. |
| Tourmaline | 7 to 7.5 | Brittle along its length; cushion well. |
| Unakite | 6 to 7 | Mixed minerals; softer spots may undercut slightly. |
Medium stones: 5.5 to 6.5
Tumble gently and separately from agate. Shorten each step, check every few days, and use plastic pellets from the first step. Our six-pack kit for hard or soft rocks gives you finer pre-polish steps.
| Material | Mohs hardness | Notes |
|---|---|---|
| Jade (nephrite) | 6 to 6.5 | Very tough. Can polish beautifully but slowly. |
| Jade (jadeite) | 6.5 to 7 | Tough; standard to gentle schedule. |
| Labradorite | 6 to 6.5 | Splits along cleavage planes; cushion well. |
| Moonstone, sunstone | 6 to 6.5 | Cleavage planes; shorter steps. |
| Pyrite | 6 to 6.5 | Heavy and brittle; tumble on its own. |
| Hematite | 5.5 to 6.5 | Polishes to a metallic shine; may stain the slurry red. |
| Rhodonite | 5.5 to 6.5 | Mixed hardness; check often. |
| Sodalite | 5.5 to 6 | Can split; check for cracks between steps. |
| Granite, basalt | About 6 | Grainy rocks; usually a satin rather than glassy finish. |
| Glass and sea glass | About 5.5 | Rounds quickly. See making sea glass. |
| Opal | 5.5 to 6.5 | Contains water and cracks easily. Not recommended for tumbling. |
Softer and brittle stones: 5 to 5.5
Short runs, lots of cushioning. Start at 120/220 if the shape is already good, keep every step short, and use plenty of plastic pellets. See how to tumble softer stones.
| Material | Mohs hardness | Notes |
|---|---|---|
| Obsidian, Apache tears | 5 to 5.5 | Volcanic glass. Chips easily, but polishes to a mirror shine. |
| Lapis lazuli | 5 to 5.5 | Often has softer calcite veins. |
| Turquoise | 5 to 6 | Often porous or stabilized. May take a waxy rather than glassy shine. |
| Apatite | 5 | Brittle; gentle schedule only. |
Very soft materials: below 5
Usually too soft for a rotary tumbler. These wear away quickly and are better polished by hand, or tumbled very briefly with fine media and lots of filler.
| Material | Mohs hardness | Notes |
|---|---|---|
| Fluorite | 4 | Splits easily along cleavage planes. Not recommended. |
| Malachite, rhodochrosite | 3.5 to 4 | Very soft. Hand polish instead. |
| Petoskey stone (fossil coral) | 3 to 4 | Traditionally hand polished. |
| Calcite, marble, limestone | 3 | Wears away quickly in grit. |
| Serpentine | 3 to 5 | Variable; test a piece first. |
| Slate | 3 to 4 | Splits into layers. |
| Coral, shells, pearl | 2.5 to 4.5 | Too soft and porous for grit tumbling. |
| Amber | 2 to 2.5 | Fossil resin. Hand polish only. |
Metals
Never tumble metal in grit. Metal is burnished, not ground. Use stainless steel shot with water and a little soap. See our steel shot guide.
| Material | Mohs hardness | Notes |
|---|---|---|
| Gold | 2.5 to 3 | Burnish with a jeweler’s mix of shot. |
| Silver, sterling silver | 2.5 to 3 | Jeweler’s mix; 1 to 4 hours. |
| Copper, bronze | About 3 | Steel shot brightens tarnish. |
| Brass (including casings) | 3 to 4 | Stainless pins reach inside casings. |
| Aluminum | 2.5 to 3 | Soft; short runs. |
| Iron, mild steel | 4 to 5 | Rinse and dry promptly to prevent rust. |
| Stainless steel | About 5.5 to 6.5 | Burnishes well with mixed shapes. |
The Mohs reference scale
The ten reference minerals, with everyday objects you can use to test a stone. If a stone scratches glass, it is about 6 or harder; if a steel knife scratches it easily, it is softer than about 5.5.
| Hardness | Reference mineral | Everyday comparison |
|---|---|---|
| 1 | Talc | Crumbles under a fingernail |
| 2 | Gypsum | Fingernail (about 2.5) scratches it |
| 3 | Calcite | Copper coin (about 3) scratches it |
| 4 | Fluorite | Iron nail (about 4.5) scratches it |
| 5 | Apatite | Glass and a steel knife (about 5.5) scratch it |
| 6 | Orthoclase feldspar | Scratches glass |
| 7 | Quartz | Scratches glass and steel easily |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum (ruby, sapphire) | Scratches almost everything |
| 10 | Diamond | Hardest natural material |
Silicon carbide grit is about 9 to 9.5 on the Mohs scale, which is why it can shape everything from agate to garnet. Not sure what you have? Read which rocks can you tumble together, or tell us about your stones and we’ll recommend a grit.