Related Clues:

• Curiosity
• Blue moon, e.g.
• Hen's tooth, e.g.
• One for the books
• One for the record books
• Exception to the rule
• Hole in one, e.g.
• Hole-in-one, e.g.
• It doesn't happen often
• It hardly ever happens
• Collector's item
• Curio
• Unusual item
• Something unusual
• Uncommon occurrence