Because of the attractive banding, and the rarity of the mineral, Blue John has commercial value and ornaments made from it are much sought after by collectors. Historically, as well as from Treak Cliff and Blue John Cavern, veins of the mineral were also worked from Old Tor Mine and another mine in Winnats Pass, and also from mines near Windy Knoll and Odin Mine, towards Mam Tor. Limited opencast extraction of Blue John took place on the top of Treak Cliff during World War II, and an incident of unofficial and illegal extraction from Old Tor Mine in the 1970s led to that mine being closed by order of H.M. Inspector of Mines, leaving Treak Cliff and Blue John Cavern as the two remaining sources of new stocks of the mineral. Both caves have a policy of supplying raw Blue John only to shops within Castleton, so the few outlets selling it elsewhere rely on old stocks.