How do I test hardness without damaging the specimen?

Test hardness by finding out which reference materials your mineral can scratch and which can scratch it, on a hidden spot, with the smallest mark that answers the question. The Mohs scale ranks ten minerals from talc (1) to diamond (10), and everyday objects such as a fingernail, a copper coin, window glass and a steel file fill in the steps between [1][2]. A careful test leaves a short scratch on the underside of a piece, and often no mark at all on the show face. This page gives the scale, the reference values two published sources assign to common tools, a step-by-step test, and the traps that make hardness readings wrong.

The ten Mohs minerals

Friedrich Mohs chose ten minerals so that each one scratches every mineral below it. The order is the same in every source [1][2]:

The numbers are ranks on a scratch test. Britannica's entry states that "each increment of one in the scale does not indicate a proportional increase in hardness": the step from calcite to fluorite is roughly a 25 percent increase, while the step from corundum to diamond is "more than 300 percent" [1]. All a 7 tells you about a 6 is that it scratches it; the scale says nothing about how much harder it is.

Everyday objects as reference points

Most collectors test with a kit of ordinary objects before they ever buy a set of hardness points. Two published sources give values for those objects, and they do not agree exactly:

The disagreement is real and it matters most for knives, because steel varies with the alloy and the heat treatment. The Notre Dame handout's 5 to 6.5 range is the honest one for an unknown blade. This page follows both sources by treating each object as a range, and recommends one habit that removes the problem: check your own tools against minerals of known hardness. Try your knife on a piece of apatite or fluorite and on a quartz crystal. Once you know that your knife scratches fluorite but not orthoclase, you know where it sits, whatever the books say.

Testing hardness step by step

1. Choose the spot. Turn the specimen over and find a flat or gently curved area on the back, the base or the matrix side of the mineral you want to test. Never test on a crystal face you want to keep, and never on a termination.
2. Make sure you are testing the right mineral. Many specimens have two or three minerals touching. Look under a loupe (a hand lens) and confirm that the grain you are about to scratch is the mineral in question, and that it is a fresh surface, not a weathered crust or coating.
3. Start soft and go up. Begin with the softest reference that might work (your fingernail), then the copper coin, then steel, then glass. Stop at the first one that leaves a scratch. Each test you skip is a mark you do not have to make.
4. Press and drag a short line. Hold the specimen firmly on a table, press the point of the reference object against it and draw a short line, no longer than you need to see it. A long, hard drag teaches you nothing a short one does not.
5. Wipe and look under the loupe. Rub the line with a dry fingertip or cloth. Britannica's test advice is to "make sure a true scratch is made, not a 'chalk' mark that will rub off" [1]. A soft tool leaves a trail of its own powder on a harder mineral. That trail wipes away; a scratch is a groove you can feel with a fingernail and see as a line of broken surface.
6. Reverse the test. If the steel did not scratch your mineral, try scratching the steel or the glass with an edge of the specimen. A mineral harder than glass bites into the glass and leaves a clean groove. This is often the gentler choice: you mark the glass, not the specimen.
7. Bracket the answer. Write the result as a range: "scratches glass, does not scratch quartz" means a hardness between about 5.5 and 7. That bracket is what an identification key needs, and it is all the precision the scale can give.

When a specimen is valuable or the mineral occurs only as small crystals on a show face, skip the test. Hardness rarely settles an identification by itself, and luster, streak, habit and locality may already have done the work.

Testing the reference against the specimen, or the specimen against the reference

Both work, and the rule is the same: the harder material scratches the softer. Use the reference object as the moving point when your specimen has a large, hidden flat area. Use an edge of the specimen on a glass plate or a streak plate when you cannot find a hidden area, or when you suspect the mineral is harder than steel. A glass plate costs little and you can scratch it as many times as you like.

Why a hardness reading can be wrong

The surface is not the mineral

Weathered rinds, iron stains and thin coatings can be much softer or harder than the mineral beneath. Test a fresh break or clean area.

The grains crumble instead of scratching

Britannica warns that fine-grained, friable or powdery samples may only loosen grains, so the test "may not measure a true surface hardness" [1]. Earthy masses and fine aggregates fool people this way all the time. Hematite is the textbook case: the Mineralogical Society of America's identification key gives its hardness as "1+ to 6½," because earthy red hematite crumbles under a fingernail while crystalline hematite resists a knife [4]. If grains fall out, you have learned how well the grains hold together, not how hard they are.

Hardness can change with direction

Some minerals are harder in one direction than another. The Mineralogical Society of America key gives calcite a hardness of 3 but notes that it "may be 2 on the top surface of prismatic crystals with a flat termination" [3]. If a result surprises you, try a second direction on the same crystal before you rule a species out.

Cleavage gives way

A mineral with good cleavage (the tendency to split along flat planes) can flake along those planes under a point and look softer than it is. Calcite, fluorite and the micas all do this. Look closely: a scratch is a groove, a cleavage failure is a step.

Safety when you scratch

Scratching and powdering a mineral makes a small amount of dust, and with most species that does not matter. With arsenic, lead, mercury and some other minerals it does. If you have any reason to think a specimen is toxic or radioactive, do not scratch or powder it, wash your hands afterward, and read how to handle and store toxic minerals first. Also keep a steady hand with knives and files, and test with the specimen resting on a table, not in your palm.

Using hardness with the other tests

A hardness bracket narrows the field fast when you combine it with luster and streak. A glassy, colorless mineral that does not scratch glass is more likely calcite or fluorite than quartz; one that scratches glass easily is more likely quartz or feldspar. The pairs that hardness separates best (quartz and calcite, fluorite and quartz, gold and pyrite) are worked through on common lookalikes, and the order in which to run all the tests is on identifying a mineral. The identification key takes your hardness range as one of its inputs.

After hardness, run the streak test

The streak plate in your kit has a second job. Rubbing a mineral across it shows the color of its powder, which is often more reliable than the color of the specimen, and the way the surface reflects light tells you its luster. Both are explained, with the minerals whose streak surprises people, on streak, luster and color.

References

1. Encyclopaedia Britannica, "Mohs hardness," revised and updated by Erik Gregersen, published 10 May 2025. https://britannica.com/print/article/387714
2. University of Notre Dame, Planet Earth laboratory, Mohs hardness scale handout (undated, read October 2026). https://www3.nd.edu/~cneal/planetearth/Lab-Mineral-ID/MOH.pdf
3. Mineralogical Society of America, The Mineral Identification Key, Table IIC-1: Streak not colored, cleavage prominent, hardness 2½ to 3 (undated, read October 2026). https://msaweb.org/msa/Collectors_Corner/id/mineral_id_keytiic1.htm
4. Mineralogical Society of America, The Mineral Identification Key, Table IIA: Minerals with a nonmetallic luster, definitely colored streak, and hardness 1 to 6 (undated, read October 2026). https://msaweb.org/msa/Collectors_Corner/id/mineral_id_keytiia.htm