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Gypsum
Gypsum
Name:Gypsum
Derivation:From "gypsos" (Greek) chalk
Formula: CaSO4.2H2O
Description: Different forms, such as Satin Spar (top left) and Desert Rose (below).

Gypsum is a source of Plaster of Paris and other plasters. It is a very soft mineral. It is a pigment, and was found in Tutankhamun's paint-box. The rose type formation is called Desert Rose. Gypsum dissolves in water, and so such crystals, which take a long time to grow, only happen in very dry conditions, such as deserts.
Selenite
Name:Selenite
Derivation:From "selene" (Greek) moon
Formula: CaSO4.2H2O
Description: Transparent diamond shaped crystals.

Selenite is the same chemical compound as gypsum. It was anciently supposed to wax and wane with the moon. This was probably because it used to drop out of any jewelry holding it. They thought this was because it got bigger and smaller, but in fact, selenite is very soft, and so would be worn away by any metal gripping it.
Alabaster is another form of Gypsum. It is semi-transluscent and often used for carving. The name comes from the Greek 'alabastros', said to be from Alabastron, a town in Egypt. But the ancients used the name Alabaster for Calcite.

Larger pictures of Gypsum:

Gypsum - Desert Rose

Gypsum

Gypsum

This is a similar colour but a different crystal formation.

Gypsum

Gypsum - Satin Spar

Gypsum

Gypsum from the cliffs in Penarth, near Cardiff

Gypsum

Cluster of Selenite crystals

Selenite

A single Selenite crystal

Selenite

Alabaster

Alabaster