Gypsum: selenite, alabaster and desert rose, a soft and soluble mineral

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What is gypsum?

Gypsum is a hydrated calcium sulphate (CaSO4·2H2O). It is so soft (2 on the Mohs scale) that it can be scratched with a fingernail. It forms mainly by evaporation of salt water, in lagoons and lakes that dry out: it is an 'evaporite' mineral, as common as it is unobtrusive. Its name comes from the Greek gypsos, 'plaster'.

It is moderately soluble in water, and when heated it loses its water: first it becomes bassanite, which is used to make plaster, then anhydrite. 'Plaster of Paris' owes its name to the gypsum quarries of Montmartre.

It is the main material of plaster, of plasterboard for partition walls (drywall) and of chalk for writing. It is also used as fertiliser, as a coagulant for tofu, and in medical splints.

The varieties: one mineral, several names

Name	Appearance	Remark
Selenite	transparent or translucent crystals	name of the clear variety (see selenite)
Satin spar	parallel fibres, silky lustre	widely sold as wands and spheres
Alabaster	fine grain, white to beige	easy to carve; not to be confused with calcareous alabaster
Desert rose	a flower of crystals trapping sand	forms in deserts

The Naica crystals in Mexico reach up to 12 m in length: a record among known crystals. The gypsum dunes of White Sands National Park in New Mexico are another spectacular example.

How does gypsum form?

When the water of a closed sea or a salt lake evaporates, the dissolved salts are deposited in a precise order, from the least soluble to the most soluble. Calcium carbonate comes first, then gypsum, then table salt (halite) and other salts. The gypsum layers formed in this way can reach several metres in thickness.

In certain warm caves and mines, such as at Naica in Mexico, sulphate-rich water deposits crystals for hundreds of thousands of years, which allows some crystals to become immense. Desert rose forms when gypsum crystallises in damp sand, enclosing grains.

Like all evaporites, gypsum preserves the trace of an ancient dry climate. It thus helps geologists reconstruct the history of vanished seas.

Traditional symbolism

The clear variety, selenite, takes its name from the Greek moon goddess, Selene. It is associated with soft light, clarity and calm. Alabaster, the material of vases, lamps and sculptures, has a long craft tradition in Egypt, Mesopotamia and Europe.

These associations are symbolic. No study shows that gypsum or selenite has any effect on mood, sleep or health.

Traditional correspondences

Area	Cited correspondence
Chakra	crown and third eye (selenite); desert rose: root or sacral
Element	air or earth
Signs	Cancer, Taurus (highly variable)
Keywords	clarity, gentleness, calm

These correspondences are given for information only, with the usual caution; see the crown chakra.

How to use it

• As decoration: alabaster lamps, spheres, bars, clusters of desert rose (see the raw stones, selenite and geodes range).
• As a stone tray: a selenite dish for placing other stones. Selenite is soft and scratches easily; do not use it to hold harder stones.
• In the studio: alabaster is carved with hand tools.
• As a contemplation stone: place a selenite in front of a soft light.

A small exercise: the fingernail test

Hardness is discovered with a simple gesture:

1. Take a piece of gypsum (selenite, alabaster) and a piece of quartz or rock crystal.
2. Press your fingernail gently on each, making a small line.
3. Look: the gypsum leaves a white mark, the quartz stays intact.
4. Wipe away the white powder with a dry cloth.

This is what '2 on the Mohs scale' means: a mineral that can be scratched with a fingernail.

Care: water is the enemy

• Water: never immerse gypsum in water or wash it under the tap. It dissolves slowly and dulls. A dry or barely damp cloth is enough, with immediate drying.
• Humidity: avoid bathrooms and cellars.
• Heat: avoid strong heat sources, which cause the crystal to lose its water.
• Knocks: gypsum is fragile.
• Salt: to be avoided.
• Dust: powdered gypsum can irritate the skin and mucous membranes. If you work it, wear a mask.
• Symbolic purification: smoke or sound (guide).

Recognising and choosing

• Fingernail test: harmless to the stone; gypsum scratches with a fingernail, whereas marble or a much harder calcite does not.
• Alabaster and marble: 'alabaster' from certain regions is in fact a carbonate, which is harder. The label 'gypsum alabaster' is precise.
• Satin spar: calcite or aragonite fibres are sometimes sold under this name; ask for the composition.
• Dye: spheres in bright colours are dyed.

Gypsum is inexpensive. For a harder and clearer stone, see rock crystal. See also real or fake stone.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Guide to stones.