rar-, rare- +

(Latin: rarus, rare, thin, loose, sparse; airy, porous, infrequent, unusual)

1. To make less dense.
2. To increase the porosity, or small openings, of something.
Something that is highly valued because of its scarcity.
1. That which is not happening or found often.
2. Something which is particularly interesting or valuable; especially, for collectors or scholars, because only a few exist.
1. The least widely known>
2. Most infrequently used or experienced.
3. The most uncommon or unusual.
4. The highest valued because it is the least known or seen.
rarity, rarities
1. The quality or state of being rare or happening very seldom.
2. Something that happens rarely or is particularly interesting or valuable because it is so unusual.
3. A rare person or thing; especially, something interesting or valued because it is so uncommon.