Mohs hardness test: a step-by-step scratch test guide
The Mohs hardness test compares how easily one material scratches another. Its ten reference minerals run from talc at 1 to diamond at 10. A harder mineral can scratch a softer one, which helps narrow down an identification when used alongside luster, crystal form, and other observations.
Mohs hardness scale: the 10 reference minerals
The scale orders minerals by relative scratch resistance. Its steps are not equal intervals: hardness 8 is not twice hardness 4. The reference values apply to minerals rather than to every rock as a whole; a rock can contain grains of several different minerals.
| Mohs | Reference mineral |
|---|---|
| 1 | Talc |
| 2 | Gypsum |
| 3 | Calcite |
| 4 | Fluorite |
| 5 | Apatite |
| 6 | Orthoclase feldspar |
| 7 | Quartz |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Diamond |
References: NPS: the Mohs hardness scale and its reference minerals; GIA: Mohs intervals are not evenly spaced.
Everyday objects give approximate comparisons
A fingernail is commonly used as a rough comparison around Mohs 2.5. Ordinary glass is often placed around 5.5. These are approximate references, not calibrated instruments. Steel varies with alloy and treatment, and coins vary with country, age, and composition. A modern coin should not automatically be treated as pure copper.
A mineral hardness kit with known reference points provides a more consistent comparison. Whichever reference you choose, record exactly what you used. A vague note such as ‘scratched by metal’ is less useful than a description of the tester and whether you observed a permanent groove.
References: NPS: approximate fingernail and glass comparisons.
How to test mineral hardness, step by step
First decide whether any damage is acceptable. Scratch tests leave marks, and many specimens should be left intact. If you proceed on an ordinary specimen you own, choose a clean, inconspicuous surface and start gently. Observe a single mineral grain where possible; testing the soft material between grains can produce a misleading result.
- Record the starting appearance and choose a known reference.
- Apply light pressure to a small spot, keeping sharp edges away from your hands and eyes.
- Wipe away loose powder and inspect the surface with a hand lens.
- Distinguish an actual groove from material rubbed onto the surface.
- Compare with another suitable reference only if more damage is acceptable; write down uncertain results as uncertain.
How to interpret a mineral scratch test result
Suppose a specimen scratches ordinary glass but is scratched by a known quartz point. That suggests a hardness between the two references, subject to the quality of the test. It does not name the specimen. Feldspars and other materials can occupy that interval, and weathered coatings can obscure the harder material beneath.
Use the result to compare candidate minerals with their other properties. A photograph can help you record appearance, but it cannot measure scratch resistance. Keeping the result with your specimen photos and locality makes your collection a better reference for a later identification.
Compare crystal shape, luster, and other identification clues →
Hardness is not toughness, value, or authenticity
Scratch resistance does not describe resistance to chipping or breaking. A relatively hard gem can still fracture or split. Hardness alone also cannot establish a stone’s identity, whether it is natural, whether it has been treated, or what someone should pay for it.
Skip scratch tests on faceted gems, jewelry, museum pieces, or anything valuable or unfamiliar. Avoid creating dust from unknown minerals. For important identifications, show the specimen to a geologist, mineral club, museum, or gemologist. Use Gem & Rock Identifier to explore a photo-based suggestion and organize your observations; use expert examination to resolve questions a photograph cannot answer.
Mineral hardness questions
What does it mean if a mineral scratches glass?
A permanent groove suggests the mineral is harder than that particular glass reference, commonly approximated at Mohs 5.5. Wipe away transferred powder before deciding that a scratch formed. Many minerals can scratch glass, so the result does not establish an identity. See how to interpret the scratch test →
Can the app measure Mohs hardness from a photo?
No. A photo-based suggestion can help you look up the reference properties of a possible mineral, but it does not measure your specimen’s scratch resistance. Compare several observable properties and use expert examination when a reliable identification matters. Record other crystal identification clues →
Sources & editorial note
Prepared with AI assistance for Rock Mineral ID, published by İlker Beri, the app developer. This educational guide draws on the references below and has not been independently reviewed by a geologist. Photo identification and simple field tests cannot replace expert examination.