What is the Mohs hardness scale?

The Mohs scale ranks minerals 1 to 10 by scratch resistance, from talc at 1 to diamond at 10. A harder mineral scratches a softer one. The scale is ordinal, not linear, so the steps are not equal sizes; diamond is far harder than corundum despite being only one step above.

How do I test hardness in the field?

Use common objects of known hardness: a fingernail is about 2.5, a copper coin about 3.5, a steel knife or glass plate about 5.5, and a steel file about 6.5. If a mineral scratches glass it is at least 5.5; if your nail scratches it, it is below 2.5.

What is streak and why does it matter?

Streak is the colour of a mineral's powder, seen by rubbing it on an unglazed porcelain plate. It is more reliable than surface colour because it stays constant even when the mineral's outward colour varies. Hematite, for example, always leaves a reddish-brown streak.

What does cleavage tell you?

Cleavage describes how a mineral breaks along planes of weak bonding, producing flat, repeatable surfaces. Mica peels into thin sheets (perfect basal cleavage), halite breaks into cubes, and quartz has no cleavage, fracturing in curved conchoidal surfaces instead.

Lustre is how a mineral's surface reflects light. Metallic lustre looks like polished metal, as in pyrite or galena, while non-metallic lustres include vitreous (glassy), pearly, greasy, resinous and adamantine (brilliant, like diamond). It is a key first clue in identification.

Common Minerals Reference

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Identifying minerals by their properties

Minerals are identified by a small set of physical properties: hardness, lustre, cleavage, streak and chemical composition. This reference lists 22 common rock-forming and ore minerals — from talc and gypsum up through quartz and feldspar to diamond — with each property and the chemical formula, plus a hardness-band selector and keyword filter to narrow a hand-specimen down.

The Mohs hardness scale in context

The cornerstone is the Mohs hardness scale, an ordered list of ten reference minerals where each scratches those below it:

1 Talc   2 Gypsum   3 Calcite   4 Fluorite   5 Apatite
6 Orthoclase   7 Quartz   8 Topaz   9 Corundum   10 Diamond

Combine hardness with lustre (metallic vs glassy), cleavage (how it breaks), and streak (powder colour on porcelain) to narrow down a specimen. The hardness band selector groups minerals as soft (1–3), medium (3.5–5.5) or hard (6–10), and the keyword filter matches name, formula, lustre or streak.

A systematic identification approach

When you have an unknown specimen in hand, work through the properties in this order — it avoids the most common confusion:

Hardness first. Use the field scratch test (see below) to place the mineral in a hardness band. This alone rules out most possibilities.
Lustre second. Metallic lustre (like polished steel) instantly separates ore minerals such as pyrite (FeS₂) and galena (PbS) from the silicates and carbonates. Non-metallic varieties include vitreous (glassy), pearly, greasy, resinous, and silky.
Cleavage vs fracture. Does the mineral break in flat, repeatable planes, or in irregular curves? Perfect cleavage (mica, calcite) vs conchoidal fracture (quartz, obsidian) is a strong discriminator.
Streak. Rub the specimen on unglazed porcelain. The powder colour is often different from the surface colour — hematite (Fe₂O₃) looks silver-grey or reddish at surface but always leaves a reddish-brown streak, which is diagnostic.
Crystal form and colour last. These are useful confirmatory clues but unreliable as primary identifiers — many minerals grow in multiple habits and occur in a wide range of colours.

Field hardness reference points

Test object	Approximate Mohs hardness
Fingernail	~2.5
Copper coin	~3.5
Steel pocketknife	~5.5
Window glass	~5.5
Steel file	~6.5
Hardened steel	~7+

If your specimen scratches glass but not a steel file, it falls in the 5.5–6.5 range — consistent with orthoclase feldspar or apatite.

Tips and notes

Mohs hardness is ordinal — diamond (10) is roughly 1,500 times harder than corundum (9) by absolute measure, despite being only one step above.
Streak is more reliable than surface colour for identification.
Quartz (SiO₂, hardness 7) has no cleavage and breaks in curved conchoidal fractures — a key distinction from feldspars, which cleave cleanly at close to 90° angles.
Calcite (CaCO₃) effervesces in dilute acid, which is a definitive chemical test in the field if you carry a small dropper of vinegar or HCl.