Mohs hardness describes how easily a mineral’s surface can be scratched. The scale runs from 1 to 10, using reference minerals from talc to diamond. A comparison can help narrow an identification, especially when two minerals look alike.
The numbers rank scratch resistance. They do not rise in equal steps, so a mineral rated 8 is not simply twice as hard as one rated 4. A hard mineral can also chip or break: resistance to those kinds of damage is called toughness. GIA explains the distinction and the scale’s uneven spacing.
In this article: reference scale · reading a result · testing cautions · glass scratches.
The Mohs hardness scale
| Mohs hardness | Reference mineral |
|---|---|
| 1 | Talc |
| 2 | Gypsum |
| 3 | Calcite |
| 4 | Fluorite |
| 5 | Apatite |
| 6 | Orthoclase feldspar |
| 7 | Quartz |
| 8 | Topaz |
| 9 | Corundum |
| 10 | Diamond |
The National Park Service’s hardness reference lists these ten minerals.
Approximate hardness of common test objects
You can also test a mineral against everyday objects such as a fingernail, a penny or a nail, as the National Park Service (NPS) notes. The University of North Georgia (UNG) geology lab manual gives values for the tools in its own kit. All of these values are approximate:
| Test object | Approximate Mohs hardness | Listed by |
|---|---|---|
| Fingernail | 2.5 | NPS, UNG |
| Copper wire | 3 | UNG |
| Copper penny | 3.5 | NPS |
| Iron nail | 4 | UNG |
| Glass plate | 5.5 | NPS, UNG |
| Knife | 5.5 | NPS |
| Steel nail | 6.5 | NPS |
The two sources give nails very different values. Use the rating supplied with your kit where you have one, and treat household objects cautiously: their composition and hardness can vary.
Read a result as a range
A useful test record says what scratched what. For example, suppose a verified Mohs 5 point leaves a groove in a sample, but a Mohs 4 point does not. You would record that the sample’s hardness appears to fall between those references.
Write down the test tool, the spot tested and what you saw after wiping the surface. A streak of loose powder can look like a scratch. If the mark wipes away and no groove remains, don’t count it as a confirmed result. The UNG geology manual cautions against confusing mineral powder with a scratch. A streak test produces that powder deliberately and reads its color. That article also covers the marks on the plate that mislead.
If different spots behave differently, the specimen may contain several materials or have an altered surface. Keep those results separate rather than averaging them into one hardness number.
For gold-colored grains in a rock, our pyrite vs gold comparison explains why scratching the surrounding rock cannot identify those grains.
Before you try a scratch test
Start with clear identification photos and observations that leave the specimen intact. Save scratch testing for an ordinary sample you are willing to mark, and follow the instructions supplied with a proper mineral hardness kit. A small, inconspicuous spot is enough; stop if the result is unclear.
Leave jewelry, polished keepsakes, coated pieces and valuable specimens unmarked. Do not grind a sample, create dust or break glass to make a test surface. The UNG manual advises laying a glass plate on a flat surface rather than holding it in your hand, in case it breaks. Watch for sharp edges on the plate and on knife blades. Ask a geology educator or mineral club for help with an unfamiliar specimen or test method.
A record might read:
Tool: verified hardness point. Spot: small exposed grain. Result: visible groove after wiping. Other areas: not tested.
Fill in the tool’s actual rating and your own observations. A record like this makes clear which part of the specimen you examined.
Why scratching glass doesn’t name the mineral
Several minerals can scratch glass. GIA notes that anything with a hardness the same as or greater than glass, about 5–6 on the Mohs scale, can do so. That result alone cannot tell you that a stone is a diamond. See GIA’s answer to the glass-scratch identification question.
Hardness becomes more useful when you combine it with other properties. Quartz and calcite, for example, differ in both hardness and cleavage; our quartz vs calcite guide puts those clues together, and our crystal-face article explains how to recognize cleavage. If you are comparing quartz with manufactured glass, read quartz vs glass before drawing a conclusion from one test.