Mohs Hardness Scale

The Mohs hardness scale ranks minerals by their resistance to scratching, from 1 (talc) to 10 (diamond). It’s an ordinal, non-linear field test: if mineral A scratches mineral B, then A is harder. You identify an unknown by trying to scratch it with reference minerals (or everyday items with known hardness) and narrowing the range. Because the steps aren’t equal—diamond is far harder than corundum even though they’re only one step apart—the scale is best for quick comparisons, not precise engineering values. Hardness also differs from toughness (resistance to breaking) and cleavage (how a mineral splits), so a hard mineral can still chip or shatter.

Hardness 3 – 4 (Soft Stones)

• Marble → Mohs ~3 (calcite-based, scratches easily)
• Sandstone → Mohs 3–4 (varies with quartz content; softer types crumble in tumbling)

[!warning] These stones are soft and not ideal for tumbling with harder materials. Tumble separately and handle gently.

Hardness 5 – 6 (Medium Hard Stones)

• Feldspar → Mohs 6 (pink, white, or green crystals in granite, unakite, gneiss)
• Epidote → Mohs 6–7 (green mineral, takes a satin polish)
• unakite → Mohs 6 (composite rock of feldspar, epidote, quartz – hardness varies)
• Conglomerate → Mohs 5–6 (depends on cement and pebbles inside)
• breccia → Mohs 5–6 (similar to conglomerate but with angular fragments)
• granite → Mohs 6–7 (mix of feldspar and quartz, usually solid enough for tumbling)
• gneiss → Mohs 6–7 (metamorphic, alternating feldspar and quartz bands)

[!tip] Medium hardness stones can be tumbled together. Expect mixed results with polish — feldspar surfaces may look satin while quartz shines.

Hardness 6.5 – 7 (Hard Stones – Excellent for Tumbling)

• quartz → Mohs 7 (clear, white, smoky varieties common)
• agate → Mohs 7 (banded microcrystalline quartz, polishes to a glass-like shine)
• jasper → Mohs 6.5–7 (opaque quartz, excellent for tumbling)
• Chert / Flint → Mohs 7 (cryptocrystalline quartz, tough and glossy when polished)
• Petrified Wood → Mohs 6.5–7 (silica replacement, takes a strong polish)
• Basalt → Mohs 6 (fine-grained volcanic rock, often smooth but not shiny)

[!tip] These are the favourite stones for tumbling. They hold up well, take a bright polish, and are the most durable in a tumbler.

Visual Reference (Compact Chart)

Notes

• Hardness is approximate — many of these rocks are composites of several minerals.
• For best results, tumble rocks of similar hardness together. Mixing soft and hard stones will damage the softer ones.
• quartz-rich stones (agate, jasper, chert) are the best tumbling candidates in this region.

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