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.
How to perform the test
- 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.
How to read the result
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 from the LithoMap guide library
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
Test safely
- 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.
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.
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
LithoMap wrote this guide in original language using the physical-property standards and safety information supported by these sources.
- Mohs Hardness Scale U.S. National Park Service
- Minerals and their physical properties U.S. National Park Service
- Geology Lesson 2: Global Building Blocks U.S. National Park Service
Editorial review: September 18, 2026.