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Samarina Nogueira F.G.A
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Today we are talking Gypsum

Gypsum is the dihydrate calcium sulphate (CaSO4 · 2 H2O) with four varieties.

  • Selenite
  • Satin Spar
  • Desert Rose
  • Alabaster (massive monoclinic variety used in ornamental sculptures)

All varieties four have the same chemical composition and crystal system (Monoclinic). But are differentiated by their appearance.

Today we will focus on the 3 varieties that are more sort after by gem collectors,

 

  • Selenite is the clear translucent variety and can have pale pastel colourations caused by inclusions and impurities. Single monoclinic crystals formed in hydrothermal deposits, eg. forming in amethyst geodes.
  • Satin Spar is the translucent, opalescent white material with a fibrous structure causing it’s Chatoyant (Cat’s eye) optical effect. Satin spar is commonly sold as Selenite which is acceptable as the gemmological definition of selenite is colourless variety of gypsum. Forms in sedimentary rocks specifically Limestone.
  • Desert Rose is clusters of Gypsum forming radiating circular crystals that look like petals of a rose. Tends to form in dry and sandy conditions.

Gypsum is only a 2 on the mohs hardness scale meaning that it is a very soft material that can be scratched by a fingernail, when scratched it will have a white powdery streak. Due to its low harness, the surface of the gypsum material is easly scratched and bruised with white percussion marks. Selenite has one excellent and two distinct cleavage directions, this means that it has a tendency to break along these structural planes because there are weaker atomic bonds in these directions. This makes it a rather brittle stone. In Roman antiquity, this ability to cleave perfectly was taken advantage of, transparent selenite was used as window panes (before the invention of glass panes), gypsum to this day is heated to remove water content to create calcium sulphate hemihydrate which is the used to make plaster. while selenite is moderately water-soluble it is a misconception that it will dissolve or become powder if placed in water. It is completely safe to rinse your selenite under water and dry with a cloth or simply wipe with a damp cloth. Salt water and detergents should not be used as they can be corrosive.

Check out my selenite mythbuster reel on instagram @fit_gemmologist