What is Derbyshire Blue John?
Blue John: A Calcium Fluoride Mineral
Derbyshire Blue John is a rare variety of fluorite, a calcium fluoride mineral.
It's geologically distinct due to its characteristic banded appearance and its extremely limited natural occurrence. Genuine Blue John can only be found in a small area near the village of Castleton, Derbyshire.
The blue, purple, yellow, and white banding is thought to come from a combination of mineral impurities, crystal growth patterns, and possible hydrocarbon inclusions. The precise causes of Blue John’s colour is still debated and not fully understood.
Blue John has been worked into decorative objects such as bowls, vases, and inlay since at least the 18th century, particularly during the Georgian period, when it became popular as an ornamental stone.
The Derbyshire Blue John vase in our photo is displayed in the Natural History Museum, London. Photograph: Stone Mania.
