Related Clues:

• Hard and firm
• Close-knit
• Mohs scale adjective
• Known by few
• Jam-packed
• Not allowing light to pass through
• Herculean
• Not translucent
• Not at all clear
• Word that can precede the first word of the six longest puzzle answers
• Thick headed
• Unbreakable, as an alibi
• Heavily populated
• Like the sea
• Not liquidy