Related Clues:

• Describing a numismatist's treasure
• Most collectible
• Least common
• Least experienced
• Like AB negative, of all major blood types
• Most unusual
• Most infrequent
• Least available
• Least seen
• Superlatively scarce
• Most elusive
• Most valuable, perhaps
• Least likely to be spotted
• Least prevalent
• Least refined