Share 'A Nearly Extinct Mineral, the Blue John'

The Blue John was used for decoration in the late eighteenth and early nineteenth centuries. It is a blue variety of fluorite, which is extracted from a Derbyshire cavern. The French called it "Bleu et jaune" and the English transcribed phonetically this name into Blue John. Its first user was Robert Adam. The deposit is now almost exhausted. The size of the ore does not exceed twenty centimeter…