Fluorite, CaF2 is also known as Fluorspar. This calcium fluoride mineral belongs to the halide family, and is a common gangue mineral in hydrothermal ore deposits. It's crystal form is usually as a cube, often with the cube corners and edges modified, or less commonly as octahedrons. This specimen is from Weardale , North Yorkshire. The light purple cubes are bedded on druzy quartz, and have obviously at one time been completely coated with marcasite, the remnants of which is showing signs of Pyrite decay.