Blue John
Blue John is a rare mineral found only in a hillside near Mam Tor, in the Derbyshire Peak District National Park, England. Blue John is calcium fluoride, colored by films of oil which were deposited on the crystals millions of years ago.
The color of Blue John is purple, banded with white, yellow and even brownish tones. The appearance is of fractured crystal, with many inclusions apparent to the naked eye. Heating the stone to a temperature of 450 degrees Fahrenheit will change the color to pinks and reds, but because of the crystal structure, it is easily fractured with heat.
Blue John is used in jewelry and decorative ware.
