Hardness Comparison

Compare the Mohs hardness of any two minerals side by side. Understand relative scratch resistance, practical hardness tests, and where minerals fall on the Mohs scale.

Analysis

Mohs Scale

How to Use

  1. 1
    Select two minerals to compare

    Choose any two minerals from the database by name or browse by Mohs hardness value from 1 to 10. The tool supports all reference minerals on the Mohs scale as well as thousands of additional species with documented hardness ranges.

  2. 2
    Review the side-by-side hardness data

    The tool displays both minerals’ hardness values, hardness ranges where applicable, practical scratch-test results using common field tools, and the relative resistance ratio. Minerals with ranges rather than point values—such as topaz (8) and kyanite (4.5–7 depending on direction)—show their full anisotropic range.

  3. 3
    Apply results to field identification

    Use the scratch-test guide to verify which field tools (fingernail, copper coin, knife blade, glass plate, steel file) can scratch each mineral. Document whether the harder mineral scratches the softer cleanly, and check that the mark is a true scratch rather than a powder streak from the softer material.

About

Hardness is among the oldest and most practically useful properties for mineral identification, described systematically since René Just Haüy’s early 19th-century crystallographic work. The Mohs hardness scale, formalized by Friedrich Mohs in 1812, defines a relative ranking of ten reference minerals—talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and diamond—that serves as a universal comparison framework used by geologists, gemologists, and materials scientists worldwide.

The physical basis for hardness lies in atomic bonding strength and crystal structure geometry. Minerals with strong three-dimensional covalent networks, such as diamond (sp³ carbon bonds) and silicon carbide, achieve extreme hardness because every bond must be overcome to displace surface atoms. Minerals with layered structures, like talc (Mg₃Si₄O₁₀(OH)₂) and graphite, are extremely soft because layers are held together only by weak van der Waals forces, allowing easy cleavage and surface abrasion. Ionic compounds with moderate bond strengths, such as halite (NaCl) and calcite (CaCO₃), fall in the middle range.

For gemologists, the Mohs scale provides the first filter for evaluating gem suitability in different jewelry applications. Stones rated below 7 risk being scratched by quartz dust, which is ubiquitous in household environments. Materials scientists use Vickers and Brinell hardness indices alongside Mohs ratings for engineering applications, as the absolute hardness values are required for machining and abrasive design calculations.

FAQ

What exactly does the Mohs hardness scale measure?
The Mohs hardness scale measures scratch resistance—specifically, the ability of one mineral to mechanically abrade the surface of another. A mineral of hardness 7 can scratch all minerals rated 6.9 or below and cannot be scratched by any mineral rated 6.9 or below. The scale is ordinal rather than linear: the hardness difference between corundum (9) and diamond (10) is roughly 4–5 times greater in absolute terms than the gap between any adjacent pair lower on the scale. For absolute hardness, the Vickers or Knoop microindentation hardness tests provide linear, load-bearing measurements in GPa, with diamond measuring approximately 10,000 kg/mm² compared to corundum’s 2,100 kg/mm².
Why do some minerals have a range of hardness values?
Hardness anisotropy occurs when atomic bond strengths differ along different crystallographic directions. Kyanite is the classic example: parallel to the long crystal axis, hardness is approximately 4.5 (easily scratched by a knife), while perpendicular to that axis it reaches 7 (scratches glass). This directional variation reflects the layered aluminosilicate structure in which Si–O and Al–O bonds run preferentially in one orientation. Fibrous minerals like stibnite and selenite gypsum also show hardness variability depending on whether you scratch along or across the fiber direction. Collector handbooks typically report both extremes to prevent misidentification during field testing.
How are common field tools calibrated to the Mohs scale?
Field tools provide quick hardness brackets without carrying a mineral reference set. A fingernail scratches minerals up to approximately 2.5 Mohs (gypsum, talc). A copper coin reaches about 3.0 (calcite). Window glass and a steel pocketknife both fall near 5.5–6.0 (apatite to orthoclase). A hardened steel file reaches 6.5–7.0 (feldspar). Silicon carbide sandpaper grit can scratch minerals to 9. These ranges are approximate and depend on the specific alloy or glass composition used, so mineralogists often carry a standard reference chip—typically quartz at 7—for calibration in the field.
Can a mineral scratch another mineral of equal hardness?
Two minerals of nominally equal Mohs hardness can scratch each other, although the result is more ambiguous than a clear difference-in-hardness test. In practice, the test produces a mix of true scratches and powder marks from both surfaces, making interpretation difficult. Equal-hardness tests also reveal surface roughness effects: a well-crystallized specimen with a smooth cleavage face may appear harder than a weathered or granular specimen of the same mineral. For equal-hardness comparisons, a streak plate test or specific gravity measurement provides more definitive discrimination than scratch testing alone.
Does Mohs hardness predict gem durability for jewelry use?
Hardness is one component of gem durability, but toughness and stability also matter. Hardness resists scratching from abrasive particles such as household dust (primarily quartz at 7), which is why gemologists recommend hardness ≥7 for rings and bracelets exposed to daily wear. Toughness, measured by fracture resistance, is independent of hardness: diamond (10) cleaves readily in four directions and can shatter from a sharp blow, while nephrite jade (6–6.5) is among the toughest gem materials due to its interlocking fibrous microstructure. Stability describes resistance to heat, light, and chemicals—kunzite fades in strong light, and emerald is sensitive to ultrasonic cleaners. Complete durability assessment requires evaluating all three properties together.
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