Orthoclase

A potassium feldspar and one of the most common minerals in granite, where it forms the pink or cream-colored blocks.

Orthoclase specimen — Photo: Didier Descouens, CC BY-SA 4.0, via Wikimedia Commons

How to identify orthoclase

Blocky pink to cream crystals in granite.
Two cleavages meeting at a right angle — flat, shiny faces.
Hardness 6 — scratches glass with effort.
No fine parallel striations (unlike plagioclase).

Often confused with

Quartz
Quartz is glassier, harder and never breaks along flat cleavage planes.
Compare side by side
Labradorite
Labradorite shows fine parallel lines and flashes of blue or green.
Compare side by side

Where it's found

Granites worldwide
Karlovy Vary, Czech Republic
Sri Lanka (moonstone)

Uses

Ceramics and glass; moonstone is a gem variety.

Rock tumbling

Can be tumbled with care.

Did you know?

Orthoclase is the Mohs reference mineral for hardness 6. Its name means 'straight break'.

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