Related Clues:

• It's noted for its cleavage
• Small well in rock
• Hardly hard rock?
• Mineral whose name is Latin for "crumb"
• Greek characters in stone and rock
• Rock hotel in auction
• Easily split layered rock
• Material in holy book not hard ultimately
• It makes granite glint
• Mineral known for perfect cleavage
• It makes flakes
• Rock fracked for gas
• Barman has his tea in The Underground - it's course and grainy
• Fracking material
• Rock that's "fracked"