Gypsum is a calcium sulphate with a chemical formula of CaSO₄·2H₂O

It crystalises in the monoclinic system. Gypsum is the standard reference species for hardness testing on the Moh's Scale of hardness and has a hardness of 2 on Moh's Scale.

Gypsum is a brittle mineral with a perfect cleavage. Crystals may be warped, bent or twisted. Gypsum may also form in groups of tabular crystals resembling roses, hence the name 'gypsum rose'. At Coober Pedy, south Australia, thick fibrous seams of gypsum are found, often adjacent to opal layers. This form of Gypsum is known as 'satin spar' and is used for carvings due to it's unique chatoyancy. When cut across the fibres and polished on the ends, it exhibits an optical illusion when placed on a printed surface. The print appears to be on the surface of the sample.

Gypsum is colourless but can be coloured by impurities and often has inclusions of other minerals. Notable examples were native silver inclusions from Broken Hill, New South Wales and Native Copper from Mt. Elliot, Queensland.

When gypsum is well crystalised and clear, it is commonly referred to as selenite. A notable locality for well formed selenite is Cave of Sword's near Niaca, Mexico.

Gypsum is by far the commonest of all the sulphate minerals and is formed as an evaporative mineral. It is frequently found in lakes and, in Mexico and Spain, huge crystals have been found in caves. Gypsum is also known as selenite and may be found in exceptional crystals in the oxidised zones of sulphide deposits.

There are many locations for gypsum in Australia, some of which are listed above, but large clear selenite crystals were found at Broken Hill, New South Wales and , recently at Mt, Elliot, Queensland.

In South Australia, selenite has been found at numerous localities including, Kimba Gap, Lake Gillies, Lincoln Park and, in the form of 'satin spar' at Coober Pedy.