Mineral Hardness Identification Guide

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, and every geologist still 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):

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:

For field geology, fingernail-copper-glass-steel covers most ID work. If you have a mineral, test it against these in order:

1. Does it scratch your fingernail? → > 2.5
2. Does it scratch a copper penny? → > 3.0
3. Does a fingernail scratch it? → < 2.5
4. Does a glass plate scratch it? → < 5.5
5. Does it scratch a glass plate? → > 5.5
6. 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, which settles it. Pyrite (FeS₂, “fool’s gold”) looks gold but has a black or greenish-black streak; real gold has a yellow streak.

Common streak colors:

Two entries in that white row surprise people every time. Hornblende and biotite both look jet black in the rock, and both leave a pale streak. Dark surface color is not dark streak, and that mismatch is precisely why the test is worth doing.

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, or how the mineral reflects light

Luster describes the appearance of light reflecting from a fresh mineral surface:

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.

Fracture: how the mineral breaks when it doesn’t cleave. Common types: conchoidal (curved, like broken glass, which is what quartz does), splintery (long fibers), uneven, earthy.

Crystal habit

How does the mineral grow? Cube? Hexagonal column? Needle? Tabular?

Diagnostic chemical tests

Some minerals have unique chemical responses:

• Calcite effervesces vigorously in dilute hydrochloric acid, fizzing hard enough to hear
• Dolomite looks identical but only fizzes once you powder it, which is the standard way to tell the two apart
• Halite tastes salty. Do not go tasting unknown minerals to find out
• Magnetite grabs a magnet hard
• Pyrite gives off a sulfurous smell when scratched against metal
• Sulfur burns with a smell you will not mistake for anything else
• Talc feels soapy
• Graphite leaves a black mark on paper, because it is pencil lead

The mineral identification flowchart

Standard order of tests for an unknown mineral:

1. Luster. Metallic or not? This is the big branch, and it splits the problem in half
2. Hardness. Gives a rough range: 1-3, 3-5, 5-7, or 7 and up
3. Streak. Often decisive, especially for the metallic ones
4. Cleavage. How many planes, and at what angles
5. Crystal habit, if any crystal faces survived
6. Specific gravity. Just pick it up
7. Chemical tests. Fizz, magnet, smell

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 test everyone forgets

How heavy does the mineral feel for its size? Specific gravity (SG) is density relative to water:

Galena (PbS) is shockingly heavy for its size, close to 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.