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Today we are talking Gypsum
Gypsum is the dihydrate calcium sulphate (CaSO4 · 2 H2O) with four varieties.
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,
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