Blue John is only found beneath Treak Cliff Hill, near Castleton, and gets its name from its distinctive colour. It is now only mined in two caverns - Treak Cliff Cavern and Blue John Cavern.

If you want to extend your day, you can explore beneath the surrounding countryside of Mam Tor at Treak Cliff Caverns, the entrance to which is conveniently located on the climb to the summit.