Colemanite

A calcium borate forming glassy crystals inside geode-like nodules in desert lake beds.

Colemanite specimen — Photo: Bureau of Mines, Public domain, via Wikimedia Commons

How to identify colemanite

Clear to white glassy crystals in nodules.
Hardness about 4.
Light in weight.

Often confused with

Calcite
Calcite fizzes in acid.
Compare side by side
Quartz
Quartz is harder.
Compare side by side

Where it's found

Death Valley, California
Bigadiç, Turkey
Boron, California

Uses

Borate ore for glass and fiberglass.

Rock tumbling

Not suitable for tumbling.

Did you know?

It's named after William Tell Coleman, whose company ran the twenty-mule team borax wagons.

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