Blue John is a rather unique type of Fluorite which is only found around the town of Castleton, England.
It is typically clear, blue, and purple banded, and makes a nice decorative object when polished.
I have seen some true masterpiece carvings – handily, Fluorite is quite a soft stone. Blue John mining has largely stopped in Castleton, as there are only limited seams that are commercially viable – as such, the specimen price has increased quite a bit.
There is often talk of new seams being found, but no surplus of material ever seems to make its way onto the market.