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.

How it works

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.

Tips and notes

Mohs hardness is ordinal — diamond (10) is far harder than corundum (9).
Field scratch points: nail ≈ 2.5, copper coin ≈ 3.5, knife/glass ≈ 5.5.
If a mineral scratches glass, it is at least 5.5 on the scale.
Streak (powder colour) is more reliable than surface colour for ID.
Quartz has no cleavage and breaks in curved conchoidal fractures.