Selenite Mineral Crystallised Gypsum
Selenite is a type of gypsum with a distinctive crystalline structure.
Gypsum is a soft mineral made up of calcium sulphate dihydrate. Dihydrate means two molecules of water. When dissolved in water calcium sulphate causes permanent hardness.
Gypsum is widely used as a fertiliser, for blackboard chalk and is the main constituent of plaster (and plaster of Paris). Selenite is crystallised gypsum.
Although satin spar and selenite are both types of gypsum, they're not exactly the same. Satin spar features long fibrous crystals that are white and opaque. It usually has a distinctive silky or pearly lustre and is often incorrectly labelled as selenite.
Selenite forms translucent or transparent colourless crystals. This mineral is so soft it can be scratched with a fingernail.
Both pieces stand without support but with selenite being so soft you don't want them to fall over. They have a vitreous lustre which means they reflect light in a similar way to glass.
Submerging selenite in water for any length of time will cause irreparable damage. To keep selenite in tip top condition it should not be allowed to come into contact with moisture. It's a good idea to keep a silica gel sachet close to where selenite is on display.
