The Mohs hardness scale is one of the most practical tools in a rockhound’s field kit. Below you’ll find a searchable table of 40+ minerals with their hardness values, appearances, and field tests — plus a “what scratches what” lookup tool.
🔍 Mineral Hardness Search
| Mineral | Hardness | Appearance | Field Test / Notes |
|---|
⚔️ What Scratches What?
Select two minerals to see which scratches which — and what everyday objects fall in between.
What Is the Mohs Hardness Scale?
The Mohs scale was devised by German mineralogist Friedrich Mohs in 1812 as a simple way to characterize the scratch resistance of minerals. The principle is straightforward: a harder material scratches a softer one. Mohs ranked ten reference minerals from softest to hardest:
| Hardness | Reference Mineral | Field Object Reference |
|---|---|---|
| 1 | Talc | Easily scratched by fingernail |
| 2 | Gypsum | Scratched by fingernail (2.5) |
| 3 | Calcite | Scratched by copper coin (3.0) |
| 4 | Fluorite | Scratched by iron nail (4.0) |
| 5 | Apatite | Barely scratches glass |
| 6 | Orthoclase | Scratches glass (5.5), scratched by file |
| 7 | Quartz | Scratches glass and steel, scratched by corundum |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum (Ruby/Sapphire) | Scratches topaz, scratched only by diamond |
| 10 | Diamond | Scratches everything — nothing scratches it |
How to Use the Mohs Scale in the Field
Step 1: Get a fresh surface
Weathered surfaces often give false low readings. Break or sand your specimen to expose fresh, unweathered material before testing.
Step 2: Test systematically
Start with your fingernail (hardness 2.5). If the mineral is not scratched, move up to a copper coin (3.0), then an iron nail (4.0), then glass (5.5), then a steel file (6.5), and finally quartz (7.0). Stop when you find the threshold.
Step 3: Check both ways
Confirm the direction. If you think glass scratches your mineral, verify by also checking whether your mineral scratches glass. Sometimes what looks like a scratch is just powder from the glass plate — wipe both surfaces clean and examine carefully.
Step 4: Read the result
If your mineral scratches glass (5.5) but is scratched by a steel file (6.5), its hardness is between 5.5 and 6.5 — which narrows the field considerably. Combine with streak and luster for a confident identification.
Common Field Testing Objects and Their Hardness
| Object | Approx. Hardness |
|---|---|
| Fingernail | 2.5 |
| Copper coin (penny) | 3.0 |
| Brass key | 3.5 |
| Iron nail | 4.0 |
| Pocketknife blade (carbon steel) | 5.5 |
| Window glass | 5.5 |
| Steel file | 6.5 |
| Piece of quartz | 7.0 |
Why Hardness Matters for Rockhounds
Hardness is diagnostic for several common identification challenges:
- Pyrite vs. gold: Pyrite is 6–6.5, gold is 2.5–3. A copper coin scratches gold but not pyrite.
- Quartz vs. calcite: Quartz is 7, calcite is 3. Calcite dissolves in acid; quartz does not. A fingernail scratches calcite; quartz scratches glass.
- Agate vs. glass: Both look similar, but agate (7) scratches glass. If your “agate” is being scratched by glass, it’s not quartz.
- Fluorite vs. quartz: Fluorite is 4, quartz is 7. A knife easily scratches fluorite but barely marks quartz.
- Topaz vs. citrine: Both can be yellow. Topaz is 8, citrine (quartz) is 7. Topaz scratches citrine.
Limitations of the Mohs Scale
The Mohs scale is ordinal, not linear. Diamond (10) is roughly 4 times harder than corundum (9) in absolute terms, but the scale doesn’t reflect this. Corundum (9) is roughly twice as hard as topaz (8). The difference between 1 and 2 is much smaller than the difference between 9 and 10.
Hardness can also vary slightly between specimens of the same mineral depending on crystal orientation, impurities, and weathering. Treat hardness as a range (±0.5) rather than an exact number.