When I visited Science Museum, and come across the earth science gallery, I found some interesting mineral rock samples This rock is labelled as calcite, 方解石, application: calcium supplements This is labelled as gypsum石膏, application: construction AFAIK, calcite ( calcium carbonate crystal)is well known for its birefringence property. However this Science Museum sample is not transparent at all. Whereas gypsum (calcium sulphate crystal) is normally known to be non-transparent. This gallery sample is a special type also named as selenite 透石膏, and usually tabular or prismatic structure , eg from Morocco: Photo from wiki btw, it would be a waste to use it for plaster or fertilizer, if this sample has such excellent optical property.