Satin Spar
Satin spar is one of the three forms of pure natural crystalline gypsum, with one other, selenite, having a very similar features, the other natural form is gypsum rock which is commonly crushed and used in the manufacture of plaster products.
In old English gypsum was referred to as spear stone (spærstn) because of its crystal spear-like projections, of which selenite and satin spar have the most notable. Satin spar crystals are much more common than selenite crystals and often confused, whilst gypsum rock is relatively common. Other forms of crystalline gypsum are known as tabular gypsum, which is normally shorter blocks of satin spar, whilst desert roses form with grains of sand and gypsum flower which are referred to as crystal habits.
Satin Spar crystals are fibrous and white, and often appear as raw rulers or wands which can be polished into palmstones or spheres. They are translucent sideways but have quite clear light transmitting qualities lengthways, similar to fibre optics, when polished at the ends. Satin Spar crystals are sometime referred to a TV rock, because of these properties, although a mineral called Ulexite might also be referred to under the same name because it has similar qualities.
Selenite crystals on the other hand form in large flat, transparent crystals, often called plates or windows, sometimes called Lapis Specularis, and were used by the Romans in the construction of specularia (sometimes referred to as specularium), including the first types of windows before the development of glass for use in panes.
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