Selenite

Selenite wind lights

Selenite is the crystallized and purest form of plaster in nature. The highly fibrous structure is special because it lets the light shine through beautifully but also reflect it. Because selenite has a very low hardness (even lower than a fingernail) it is easy to work with. Hence the relatively low price.
Selenite occurs in a white and salmon-colored variety. Both are from Morocco.
In the past, many plate-shaped selenites were found in southern Spain, which was already used as windows in churches in the Middle Ages. This species is often called "Mariaglas".