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Hardness Test identification and geology

The hardness test measures a mineral's resistance to scratching, ranked on the Mohs scale from 1 talc to 10 diamond . Hardness is a fundamental physical property determined by the strength and density of atomic bonds within a mineral's crystal lattice. Because hardness is relatively consistent for a given mineral species regardless of specimen size or color, it is one of the most reliable and frequently used diagnostic tests in mineral identification. The Mohs scale is a relative ordinal scale, meaning each reference mineral can scratch those below it and be scratched by those above it, but the intervals between ranks are not equal. The actual measured hardness absolute hardness, or Vickers hardness increases exponentially toward the upper end of the scale—diamond is roughly four times harder than corundum despite being only one Mohs number higher. Understanding this nonlinear relationship is important when interpreting borderline results. In practice, most field geologists carry a set of common reference tools rather than a full Mohs kit: a fingernail 2.5 , a copper coin 3.5 , a steel nail or knife blade 5.5 , a glass plate 5.5–6 , and a streak plate of unglazed porcelain 6.5–7 . By systematically testing the unknown mineral against these references—and vice versa—you can bracket the hardness to within half a Mohs number in most cases. Tips: Always test on a fresh surface—weathering rinds and altite coatings can be softer than the true mineral. Use a hand lens 10× to verify scratches. Be cautious with valuable specimens; test on an inconspicuous edge. Aggregates of fine crystals e.g., hematite masses may appear softer than individual crystals because grains pluck out rather than resist scratching. Hardness picks give more precise results than improvised tools.. Equipment: Mohs hardness kit or common substitutes: fingernail 2.5 , copper coin 3.5 , steel nail or knife blade 5.5 , glass plate 5.5–6 , streak plate 6.5–7 , quartz crystal 7 . Optional: hardness picks calibrated metal points at each Mohs level. Procedure: 1. Select a fresh, unweathered surface on the unknown mineral specimen. 2. Attempt to scratch the mineral with your fingernail. If it scratches, hardness is less than 2.5. 3. If the fingernail fails, try a copper coin. A visible scratch indicates hardness below 3.5. 4. Proceed to a steel nail or knife blade. If it scratches the mineral, hardness

Sources, scope & corrections

These authoritative resources support category-level nomenclature, terminology, identification methods, and safety. When a specimen-specific safety warning appears, the public-health source listed here directly supports that precaution. They are not line-by-line citations for every other property on this entry, and natural specimens vary.

Editorial owner
GeoMiner
Last updated
2026-03-30T15:00:37.048321+00:00
Review status
Published; not independently peer-reviewed
  • How Do Geologists Identify Minerals? — Utah Geological Survey. Practical guidance on diagnostic physical properties and identification tests.
  • Mineral ID Key — Mineralogical Society of America. Detailed test methods, repeatability guidance, and safety cautions.

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Common questions

What is the Hardness Test used for?
The hardness test measures a mineral's resistance to scratching, ranked on the Mohs scale from 1 talc to 10 diamond . Hardness is a fundamental physical property determined by the strength and density of atomic bonds within a mineral's crystal lattice. Because hardness is relatively consistent for a given mineral species regardless of specimen size or color, it is one of the most reliable and frequently used diagnostic tests in mineral identification. The Mohs scale is a relative ordinal scale, meaning each reference mineral can scratch those below it and be scratched by those above it, but the intervals between ranks are not equal. The actual measured hardness absolute hardness, or Vickers hardness increases exponentially toward the upper end of the scale—diamond is roughly four times harder than corundum despite being only one Mohs number higher. Understanding this nonlinear relationship is important when interpreting borderline results. In practice, most field geologists carry a set of common reference tools rather than a full Mohs kit: a fingernail 2.5 , a copper coin 3.5 , a steel nail or knife blade 5.5 , a glass plate 5.5–6 , and a streak plate of unglazed porcelain 6.5–7 . By systematically testing the unknown mineral against these references—and vice versa—you can bracket the hardness to within half a Mohs number in most cases.
How do you perform the Hardness Test?
1. Select a fresh, unweathered surface on the unknown mineral specimen. 2. Attempt to scratch the mineral with your fingernail. If it scratches, hardness is less than 2.5. 3. If the fingernail fails, try a copper coin. A visible scratch indicates hardness below 3.5. 4. Proceed to a steel nail or knife blade. If it scratches the mineral, hardness is below 5.5. 5. Test against a glass plate. If the mineral scratches the glass, its hardness exceeds 5.5. 6. Try scratching a streak plate unglazed porcelain, 6.5 . If the mineral scratches the plate, hardness exceeds 6.5. 7. For hardnesses above 7, use known reference minerals from a Mohs kit quartz, topaz, corundum . 8. Always confirm by reversing the test: try scratching the reference material with the unknown mineral. 9. Brush away powder and examine the surface with a hand lens to confirm a true scratch groove in the surface versus a powder streak left behind.
How do you interpret the Hardness Test?
A true scratch leaves a permanent groove that cannot be rubbed away. If the mark wipes off, it is a powder residue, not a scratch—the tested surface is harder than the tool. When both materials scratch each other, they have approximately equal hardness. Hardness can vary slightly by crystal face in some minerals kyanite is a classic example: 5 along the length of its bladed crystals and 7 across the width . Record directional hardness if anisotropy is suspected.
What equipment is needed for the Hardness Test?
Mohs hardness kit or common substitutes: fingernail 2.5 , copper coin 3.5 , steel nail or knife blade 5.5 , glass plate 5.5–6 , streak plate 6.5–7 , quartz crystal 7 . Optional: hardness picks calibrated metal points at each Mohs level
What safety precautions apply to the Hardness Test?
The listed safety level is caution.

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