# Mohs hardness test

Canonical page: https://lithomap.app/resources/field-tests/mohs-hardness-test
Last reviewed: September 18, 2026
Page purpose: Original LithoMap field-method reference for human readers, search systems, and language models.

## Direct answer

Try to scratch a clean, inconspicuous surface with a reference object, then reverse the comparison when practical. A real scratch remains after loose powder is wiped away. Record a bracket—for example, harder than copper but softer than glass—instead of forcing an exact Mohs number.

## Why the test works

Hardness is resistance to scratching, not resistance to breaking. The Mohs scale orders reference minerals from talc at 1 to diamond at 10, but the numbered intervals are not equal physical increments.

The test is most useful as a bracket. If a specimen scratches one reference but is scratched by the next harder reference, its hardness lies between them.

## Field kit

- Clean hand sample with an inconspicuous test area
- Fingernail (about 2.5)
- Uncoated copper piece (about 3)
- Plain steel point or blade (roughly 5 to 5.5)
- Uncoated glass plate (about 5.5)
- Hand lens and eye protection

## Procedure

1. Expose a representative surface: Brush away loose dirt. Avoid a weathered rind, mineral coating, crystal face, polished gem, or valuable display surface.
2. Start with a softer reference: Press a reference edge across the specimen once with steady, moderate force. Do not saw repeatedly or bear down hard enough to crush grains.
3. Wipe and inspect: Remove loose powder. A groove cut into the specimen indicates a scratch; a removable gray or metallic line may only be material rubbed off the reference.
4. Reverse the comparison: Where safe, try the specimen against the reference. Reciprocal testing helps reveal a powder mark or a weak aggregate that crumbled instead of scratching.
5. Record a range: Write the two nearest reliable bounds and the surface tested. Repeat on another fresh spot before combining the result with streak, luster, cleavage, and context.

## Interpreting results

- Scratched by a fingernail: Hardness is below about 2.5; talc and gypsum are common possibilities, but a soft coating can mask a harder core.
- Not scratched by a fingernail; scratched by copper: Hardness is roughly 2.5 to 3. Calcite lies near the upper part of this interval.
- Scratches copper; does not scratch glass: Hardness is broadly between about 3 and 5.5. Use other properties to narrow the possibilities.
- Reliably scratches glass: Hardness exceeds about 5.5 at the tested point. Confirm the specimen cut the glass rather than leaving a removable streak.

## Examples and related guides

- Gypsum: Usually scratched by a fingernail; Mohs 2. — https://lithomap.app/resources/mineral-identification/gypsum
- Calcite: Harder than a fingernail but softer than copper or steel; Mohs 3. — https://lithomap.app/resources/mineral-identification/calcite
- Quartz: Scratches ordinary glass; Mohs 7. — https://lithomap.app/resources/mineral-identification/quartz
- Kyanite: Hardness changes with direction, so a single number can mislead. — https://lithomap.app/resources/mineral-identification/kyanite

## Common mistakes

- Mistaking a removable powder line for a cut in the surface
- Testing only a weathered crust or soft alteration product
- Using plated, coated, or unusually hardened reference objects
- Applying so much force that a granular rock crushes
- Treating the Mohs numbers as a linear measurement or as proof of identity

## Safety and limitations

- Wear eye protection when testing brittle pieces.
- Do not scratch a gem, polished surface, museum specimen, cultural object, or material you do not own or have permission to test.
- Avoid creating dust; never test fibrous or potentially hazardous material destructively.
- A scratch test narrows possibilities. It does not establish value, origin, or collecting authority.

## Frequently asked questions

### Does scratching glass prove a specimen is quartz?

No. Many minerals are harder than common glass, and some glass varies in hardness. Use fracture, cleavage, habit, streak, density, and geologic context as independent checks.

### Why did the specimen leave a line but not scratch?

A softer object can rub off on a harder surface. Wipe the line away and inspect with magnification; a true scratch is a persistent groove.

### Can I test a crystal face?

Avoid it. A crystal face may be valuable, coated, or unrepresentative, and the test can permanently damage it. Use an inconspicuous broken surface only when you have permission.

## Authoritative sources

- Mohs Hardness Scale — U.S. National Park Service: https://www.nps.gov/articles/mohs-hardness-scale.htm
- Minerals and their physical properties — U.S. National Park Service: https://www.nps.gov/subjects/geology/minerals.htm
- Geology Lesson 2: Global Building Blocks — U.S. National Park Service: https://www.nps.gov/teachers/classrooms/geology-lesson-2-global-building-blocks.htm

This method supports preliminary field identification. It is not laboratory confirmation, appraisal, hazard clearance, proof of origin, or permission to collect or alter a specimen.