Fluorite, calcium fluorite,
Formula CaF₂.

Crystal system: Cubic.
Hardness: 4.0
Density: 3.2
Colour: Very variable, colourless, green, blue, violet, yellow, pink.
Streak: Colourless.

[Fluorescent]

Fluorite is the best known of all fluorescent minerals, and surely features in all fluorescent mineral collections. It was the bluish tinge, seen in strong sunlight, on fluorites from the Lake District and Durham in the nineteenth century that started the interest in fluorescence and gave the phenomenon its name. Most fluorites fluoresce a bright mauvish colour in both long wave and short wave ultraviolet radiation.

This specimen in the picture is fluorite from Weardale, County Durham and is shown fluorescing under short wave ultraviolet radiation. The red fluorescing mineral has not been identified but gives an attractive combination of colours.