Mineral Hardness Identification Guide
Identify minerals by Mohs hardness (1-10), streak color, and luster.
Includes household scratch tests for each hardness level and a table of 30 minerals.
The Mohs hardness scale
Friedrich Mohs developed his hardness scale in 1812 by ranking 10 common minerals from softest to hardest based on which could scratch which. It’s been the gold standard for mineral identification ever since — every geologist learns it. The scale is relative, not linear: each step represents a different absolute hardness, with the top end (diamond) being many times harder than #9 (corundum) in absolute terms.
The classic Mohs scale (Mohs 1812 / modernized):
| Hardness | Mineral | Composition |
|---|---|---|
| 1 | Talc | Mg₃Si₄O₁₀(OH)₂ |
| 2 | Gypsum | CaSO₄·2H₂O |
| 3 | Calcite | CaCO₃ |
| 4 | Fluorite | CaF₂ |
| 5 | Apatite | Ca₅(PO₄)₃(F,Cl,OH) |
| 6 | Orthoclase (feldspar) | KAlSi₃O₈ |
| 7 | Quartz | SiO₂ |
| 8 | Topaz | Al₂SiO₄(F,OH)₂ |
| 9 | Corundum | Al₂O₃ (ruby, sapphire) |
| 10 | Diamond | C |
The famous test: a mineral can scratch anything softer than itself. So if mineral X scratches calcite (3) but is scratched by fluorite (4), its hardness is between 3 and 4 (typically reported as 3.5).
The household reference scale
The genius of Mohs’ scale is that you can test it with common objects:
| Reference object | Hardness |
|---|---|
| Soft talc / soapstone | 1 |
| Fingernail | 2.5 |
| Copper penny (pre-1982 US) | 3.0 |
| Tooth enamel | 5.0 |
| Glass plate | 5.5 |
| Iron nail | 4.0 (varies; can be 5.5 if hardened) |
| Steel file or hardened knife | 6.5 |
| Streak plate (unglazed porcelain) | 7.0 |
| Quartz (any clear/milky river pebble) | 7 |
| Sapphire crystal (watch face) | 9 |
| Diamond | 10 |
For field geology, fingernail-copper-glass-steel covers most ID work. If you have a mineral, test it against these in order:
- Does it scratch your fingernail? → > 2.5
- Does it scratch a copper penny? → > 3.0
- Does a fingernail scratch it? → < 2.5
- Does a glass plate scratch it? → < 5.5
- Does it scratch a glass plate? → > 5.5
- Does a steel knife scratch it? → < 6.5
A few quick tests narrow most minerals to a hardness range.
The streak test
The streak is the color of a mineral’s powder, often different from the mineral’s surface color. The standard test: scratch the mineral firmly across an unglazed porcelain tile (a “streak plate”). The resulting powder shows the streak color.
Why does streak matter? Surface color can be deceiving. Hematite (Fe₂O₃) varies from silver to red to black on the surface, but always has a distinctive blood-red streak — diagnostic. Pyrite (FeS₂, “fool’s gold”) looks gold but has a black or greenish-black streak; real gold has a yellow streak.
Common streak colors:
| Streak color | Common minerals |
|---|---|
| White / colorless | Quartz, feldspar, calcite, gypsum, talc, fluorite, mica |
| Black | Magnetite, pyrite, hornblende, biotite |
| Red / red-brown | Hematite |
| Yellow | Sphalerite, sulfur, limonite |
| Green | Chlorite, malachite |
| Blue | Azurite |
| Gray | Galena, graphite |
| Brown | Sphalerite (some varieties), siderite |
Limitation: minerals harder than 7 (quartz, topaz, corundum, diamond) usually won’t streak on a standard 7-hardness porcelain plate. Powdering by another method is needed.
Luster — how the mineral reflects light
Luster describes the appearance of light reflecting from a fresh mineral surface:
| Luster type | Description | Examples |
|---|---|---|
| Metallic | Shiny like polished metal | Pyrite, galena, gold, magnetite, chalcopyrite |
| Vitreous / glassy | Like broken glass | Quartz, fluorite, feldspar, calcite, garnet |
| Adamantine | Brilliant, sparkly | Diamond, zircon, cerussite |
| Resinous | Like dried tree sap | Sphalerite, sulfur, amber |
| Pearly | Iridescent like pearl | Talc, gypsum, muscovite, biotite |
| Silky | Long parallel fibers reflecting light | Asbestos, satin spar gypsum, malachite |
| Greasy / oily | Looks coated in oil | Nepheline, opal (some), serpentine |
| Dull / earthy | No noticeable reflection | Kaolinite, bauxite, limonite |
| Submetallic | Between metallic and non-metallic | Hematite (some varieties), magnetite |
Metallic luster is the easiest to identify — if a mineral looks like metal, it almost certainly is metallic. Non-metallic minerals are then subdivided by the specific way they reflect light.
Cleavage and fracture
Beyond hardness, streak, and luster, two more properties matter for ID:
Cleavage: the tendency to break along smooth, flat planes. Some minerals have perfect cleavage in specific directions (mica splits into sheets; fluorite into octahedral pieces; halite into cubes). The number and angles of cleavage planes are diagnostic.
| Cleavage type | Examples |
|---|---|
| 1 perfect direction | Mica, talc, graphite |
| 2 directions at 90° | Galena (cubic), halite (cubic) |
| 2 directions ~120°/60° | Hornblende, amphiboles |
| 3 directions at 90° (cubic) | Galena, halite, pyrite (poor) |
| 3 directions not 90° (rhombohedral) | Calcite, dolomite |
| 4 directions (octahedral) | Fluorite, diamond |
| No cleavage | Quartz, garnet (these “fracture” instead) |
Fracture: how the mineral breaks when it doesn’t cleave. Common types: conchoidal (curved, like glass — quartz), splintery (long fibers), uneven, earthy.
Crystal habit
How does the mineral grow? Cube? Hexagonal column? Needle? Tabular?
| Habit | Description | Examples |
|---|---|---|
| Cubic | Six equal square faces | Pyrite, galena, halite, fluorite |
| Octahedral | Eight triangular faces | Fluorite (sometimes), spinel |
| Hexagonal | Six-sided columns | Quartz, beryl, apatite |
| Prismatic | Long crystals with parallel sides | Tourmaline, hornblende |
| Tabular | Flat plates | Mica, gypsum |
| Botryoidal | Like a bunch of grapes | Malachite, hematite (sometimes) |
| Massive | No crystal form visible | Most rocks |
| Acicular | Needle-like | Stibnite, natrolite |
| Dendritic | Tree-like branching | Native copper, manganese oxides |
Diagnostic chemical tests
Some minerals have unique chemical responses:
- Calcite — effervesces (fizzes) vigorously in dilute HCl
- Dolomite — only fizzes when powdered
- Halite — tastes salty (don’t taste unknown minerals!)
- Magnetite — strongly attracts a magnet
- Pyrite — produces sulfurous smell when scratched against metal
- Sulfur — burns with characteristic smell
- Talc — feels soapy
- Graphite — leaves a black mark on paper (it’s literally pencil lead)
The mineral identification flowchart
Standard order of tests for an unknown mineral:
- Luster — metallic vs non-metallic? Major branch
- Hardness — gives a rough range (1-3, 3-5, 5-7, 7+)
- Streak — often diagnostic, especially for metallic minerals
- Cleavage — number of planes and angles
- Crystal habit — if visible
- Specific gravity — feel the heft
- Chemical tests — fizz, magnet, taste (cautiously)
Combine these and most common minerals can be identified in under 5 minutes. Definitive identification of rare or unusual minerals often requires lab analysis (X-ray diffraction, electron microprobe, etc.).
Why Mohs scale has limitations
The Mohs scale is relative, not linear:
- Diamond (10) is about 4x harder than corundum (9) by absolute measurement
- Corundum (9) is about 2x harder than topaz (8)
- Quartz (7) is about 2x harder than feldspar (6)
- Talc (1) and gypsum (2) differ by only ~5x
Engineers use Vickers or Knoop hardness for precise measurements (in GPa or kg/mm²). Mohs is for field ID; precision hardness measurements use better methods.
Specific gravity — the often-overlooked test
How heavy does the mineral feel for its size? Specific gravity (SG) is density relative to water:
| Mineral | SG |
|---|---|
| Most rocks | 2.5 - 3.0 |
| Quartz | 2.65 |
| Calcite | 2.71 |
| Feldspar | 2.55 - 2.76 |
| Mica | 2.7 - 3.2 |
| Hematite | 5.3 |
| Magnetite | 5.2 |
| Galena | 7.5 (very heavy!) |
| Pyrite | 5.0 |
| Native gold | 19.3 (extremely heavy) |
| Native copper | 8.9 |
Galena (PbS) is shockingly heavy for its size — feels like solid lead. Gold is even more so. Specific gravity is a quick “weight test” with your hand, useful for picking out metal-rich minerals from common silicates.
Bottom line
Mohs hardness, streak, luster, and cleavage form the foundation of mineral identification. Most common minerals (quartz, feldspar, mica, calcite, hornblende, magnetite, pyrite) can be ID’d in the field with just these tests and a basic kit. Definitive identification of rare specimens needs lab analysis, but for routine geology and crystal collecting, the 200-year-old Mohs system is still the working tool.