Alabaster is a massive, fine-grained form of gypsum (calcium sulfate), usually translucent and white. It has been used ornamentally for many centuries, being easily carved. In the Middle Ages alabaster was cut into thin laminae for use in church windows, notably in Italy, e.g., San Vitale, Ravenna. It can also be dyed or made opaque by heating. Ancient oriental alabaster was a yellowish marble.