The ten steps of the Mohs scale
| Hardness | Mineral | Where you meet it |
|---|---|---|
| 1 | Talc | Soapstone is mostly talc, and a fingernail scratches it. |
| 2 | Gypsum | Alabaster is gypsum. A fingernail scratches it. |
| 3 | Calcite | Marble and limestone are mostly calcite. |
| 4 | Fluorite | Iron is also about 4. |
| 5 | Apatite | Knife blades and steel files run from 5 to 6.5. |
| 6 | Orthoclase | A feldspar and a main component of granite. |
| 7 | Quartz | Rock crystal, amethyst and rose quartz are quartz. |
| 8 | Topaz | Scratches quartz. |
| 9 | Corundum | As a gemstone, sapphire and ruby. |
| 10 | Diamond | Scratches all others. |
How to do a Mohs hardness test at home
The rule behind it: if A scratches B, A is harder. If neither scratches the other, they are about equal. If B scratches A but C does not, A falls between B and C. With an unknown specimen, the scratch test narrows the hardness to a range.
- Pick a clean, smooth spot on the specimen that nobody will see, and hold it firmly against a table.
- Try a fingernail. If it leaves a mark, hardness is below about 2.5.
- Try copper, then a pocketknife or a piece of glass. Press firmly and drag the point once across the surface. Write down what scratches and what does not.
- Brush the spot clean and look at it with a hand lens. A scratch is a groove in the rock. A mark that wipes off is residue from your tool.
- Repeat the test to confirm, then record the hardness as a range, for example “between 3 and 5.5”.
Safety Do not scratch jewelry, collector pieces or paintings. Do not scratch fibrous or dusty materials, and do not breathe dust.
Hardness of everyday tools
Household tools work for a hardness test if you treat their hardness as a range. Values vary with the material, and sources differ: Wikipedia puts glass at 5.5, while geology.com gives 4 to 7.
| Tool | Hardness | Note |
|---|---|---|
| Fingernail | 2 to 2.5 | Most fingernails fall between 2 and 2.5. |
| Copper wire or sheet | 2.5 to 3 | Use real copper. US pennies made since 1982 are zinc with a thin copper layer, and earlier ones were 95 percent copper, so a penny is an unreliable stand-in. |
| Nail | 4 to 7 | Depends on the metal. Pure iron is about 4, ordinary steel 4 to 4.5. |
| Glass | 4 to 7 | Window glass is often near 5.5. Other glass can be far from that. |
| Knife blade | 5 to 6.5 | Depends on the steel. |
| Steel file | 5 to 6.5 | Not every file is made of steel. |
| Porcelain streak plate | 5 to 7 | Plates from several vendors fell in this range. |
Mohs hardness picks work on the same principle. They have a metal tip of known hardness, from 2 (plastic) and 3 (copper) up to 9 (alloys), and you drag the tip across the specimen.
How to read your result
| Result | Hardness | Examples |
|---|---|---|
| A fingernail scratches the rock | up to about 2.5 | Talc, gypsum |
| Copper scratches it, a fingernail does not | about 2.5 to 3 | Biotite (2.5 to 3), calcite sits at the edge |
| Copper does not scratch it, a nail does | about 3 to 4 | Malachite (3.5 to 4), fluorite (4) |
| A knife blade scratches it | up to about 5 or 6 | Apatite (5) |
| Neither knife nor glass scratches it | from about 6 to 6.5 | Orthoclase (6 to 6.5), quartz (7) |
These are guide values because tools vary. Use the result to narrow down the options.
Mohs hardness of common minerals and gemstones
Where sources differ, the table gives a range. Sources are listed at the end of the page.
| Stone | Hardness | Note |
|---|---|---|
| Malachite | 3.5 to 4 | About as hard as calcite to fluorite. |
| Opal | 5.5 to 6 | Softer than quartz. |
| Pyrite | 6 to 6.5 | About as hard as orthoclase. |
| Agate | 6.5 to 7 | A quartz variety, often banded. |
| Jadeite | 6.5 to 7 | About as hard as agate. |
| Garnet | 6.5 to 7.5 | Sources differ: geology.com 6.5 to 7.5, Wikipedia 7 to 7.5. |
| Quartz | 7 | Includes rock crystal, amethyst and rose quartz. |
| Emerald | 7.5 to 8 | Harder than quartz. |
| Sapphire | 9 | Corundum. |
| Diamond | 10 | The top of the scale. |
Granite is a rock made of several minerals. Some sources give it 6 to 7, and the number depends on its mix of minerals.
The hardest and softest minerals
Talc is the softest mineral on the scale, and diamond is the hardest at 10. Diamond can still chip. Hardness measures resistance to scratching. A hard material can break under stress, which is a lack of toughness, and a diamond can be cleaved along certain planes.
On the Mohs scale quartz is 7 and diamond is 10. Granite at 6 to 7 sits above ordinary steel at 4 to 4.5.
Why the scale is not linear
Mohs picked ten minerals and numbered them 1 to 10. The numbers form a ranking. Wikipedia lists the absolute hardness measured with a sclerometer as talc 1, gypsum 2, calcite 14, quartz 100, corundum 400 and diamond 1500. Diamond is about four times as hard as corundum, and corundum about twice as hard as topaz.
That is why you should not add or average Mohs numbers like distances. Mohs introduced the scale in 1812 in a book on identifying fossils, and he picked minerals that were easy to get, apart from diamond.
What hardness cannot tell you
Hardness narrows the list; it does not name the mineral. Quartz and chalcedony have almost the same hardness. Streak, luster, cleavage and density matter too. Luster describes how a surface reflects light, for example metallic or glassy. Streak is the color of the powder. Cleavage is how a mineral breaks along flat planes. For the full routine, see identify a rock by photo.
Hardness in LithoCanvas
LithoCanvas shows hardness, crystal system, color and luster together, with an estimated confidence. The app has a Mohs hardness test and a glossary of the terms.
Sources
Scale, order, tool hardness and test procedure: https://geology.com/minerals/mohs-hardness-scale.shtml
Example values (opal, jadeite, garnet, emerald, sapphire, glass, steel, granite) and absolute hardness: https://en.wikipedia.org/wiki/Mohs_scale
Soapstone talc content: https://en.wikipedia.org/wiki/Soapstone. Agate 6.5 to 7: https://en.wikipedia.org/wiki/Agate. Quartz 7: https://www.nps.gov/subjects/geology/minerals.htm
US cent composition: https://en.wikipedia.org/wiki/Lincoln_cent
Marble in sculpture practice: https://www.bildhau.de/de/die-haerte-von-steinen
Questions
- What does Mohs hardness 7 mean?
- The mineral scratches glass and is scratched by topaz and corundum. Quartz is 7, and so are rock crystal, amethyst and rose quartz.
- Is the Mohs scale linear?
- No. It is a ranking, and the gaps between steps are not equal. Diamond (10) is about four times as hard as corundum (9) although only one step separates them.
- What is the Mohs hardness of a fingernail, a penny and a knife?
- A fingernail is 2 to 2.5 and a knife blade is 5 to 6.5. A penny is unreliable: US pennies made since 1982 are zinc with a copper layer, and earlier ones were 95 percent copper. Use a piece of real copper, which is 2.5 to 3.
- How do I test the hardness of a mineral at home?
- Scratch an inconspicuous spot in turn with a fingernail, copper, and a knife or glass. Brush the spot clean, check with a hand lens whether a groove formed, and record the result as a range. Hold the specimen firmly, press hard and repeat the test once to confirm.
- What are the hardest minerals?
- Diamond is the hardest at 10, followed by corundum at 9, which includes ruby and sapphire, and topaz at 8. Talc at 1 is the softest.
- Is a harder stone always stronger?
- No. Hardness is resistance to scratching. A hard material can still break under stress, and diamond can be cleaved along certain planes.
- What can be mistaken for quartz?
- Calcite and feldspar are common minerals to compare against: calcite is 3, orthoclase feldspar 6 to 6.5, quartz 7. Chalcedony and agate are very fine-grained quartz at 6 to 7, so hardness alone does not separate them.
- Can I test gemstones this way?
- Only to a limit, and never on a piece of jewelry. The test leaves marks. For gemstones, ask an appraiser.
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