Related Clues:

• It's found underground
• Promising rock
• See 37 Across
• Unrefined find
• Mt. Hood's state
• Assay subject
• Unwrought iron
• Nugget holder
• Blast furnace material
• Buried treasure
• Volcanic stuff
• Driller's discovery
• Prospecting find
• Silver source
• Boxcar cargo, perhaps