silicates
Orthoclase
K(AlSi3O8)
- Also known as
- potassium feldsparK-feldspar
- Crystal system
- monoclinic
- Hardness
- 6 on the Mohs scale
- Lustre
- vitreous
- Colour
- White, cream, pale pink, occasionally colourless
- Where it forms
- One of the commonest minerals in the earth’s crust, crystallising from granitic magma. Feldspars make up around sixty per cent of the crust by volume.
- Chiefly from
- MadagascarMyanmarworldwide in granite
Safety
No hazard from handling.
Where it comes from
Quarried in enormous quantity as an industrial mineral rather than a gem — feldspar is a principal ingredient of ceramics and glass, and world production runs to tens of millions of tonnes a year.
Historical use
Defines position 6 on the Mohs hardness scale, which Friedrich Mohs published in 1812 using ten common minerals that could be told apart by scratching. Industrially it matters far more than any gem in this collection.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
Worth knowing
Orthoclase is here because the two stones that follow it are its varieties, and because it makes the scale of the whole subject clear. Feldspars are about sixty per cent of the earth's crust. The entire crystal trade, every stone in every shop, is drawn from a thin slice of unusual material sitting on top of a planet made mostly of this.
The name means "straight fracture", from its two cleavage planes meeting at ninety degrees. That right angle is how you tell a feldspar from a quartz at a glance: quartz has no cleavage and breaks in curved shells, feldspar breaks in flat steps.
Identity checked against IMA-CNMNC master list, June 2025.