rarity

noun [抽象名词]罕见的人（或物），珍品；稀有，罕见；稀薄

The rarity of something is the fact that it is very uncommon.
稀有•  It was a real prize due to its rarity and good condition. 它是一件真正的稀世珍品，因为它不但稀有而且保存完好。