Related Clues:

• Thin layered rock
• Mineral used for insulation
• One source of oil
• Constituent of biotite
• Not exactly hard rock?
• Stratified rock
• Mudstone
• It comes in sheets
• Possible source of petroleum
• Translucent mineral
• Material in sheets
• Sparkly rock substance
• Flexible mineral
• Laminated rock
• Petroleum source