Related Clues:

• Vein pursuit?
• Graded materials
• It's picked out of a mine
• Rich rocks
• Refinery needs
• Something to be assayed
• 45D homonym
• Mined rocks
• Zincite, e.g.
• "Buried" treasure found in 17-, 27-, 44- and 59-Across
• Metal containers?
• Bauxite, for example
• Gold and silver, e.g.
• Gold or silver sources
• Bauxite et al.