Fluorspar or fluorite.
© Science Museum / Science & Society Picture Library
A mineral consisting of calcium fluoride (CaF2) which crystallizes in the cubic system, commonly in simple cubes. Occasionally colourles, yellow or green, but typically purple (as in the picture). The coloured varieties may fluoresce strongly in ulraviolet light. The term fluorescence got its name because the phenomenon was first observed in fluorspar.