The name of the stone, Blue John, comes from its colours blue and yellow and the fact that there were many french miners working in the mines. In french blue is bleu and yellow is jaune 🙂 linguistics is fun! Blue John is only mined in 4 places in the world, all in this area and they will soon be fully excavated. If I remember correctly, the guide said that they estimated that this cave would be only for guided tours in 20 years.