(simile) extremely uncommon

The hypothesis that the evolution of life required an improbable combination of astrophysical and geological events and circumstances, and for this reason complex extraterrestrial life is a very improbable phenomenon and likely to be extremely rare.

plural of rare earth magnet

Alternative form of rare earth element

(simile) Exceptionally rare.

(simile) Extremely rare or scarce.

To make rare, thin, porous, or less dense / To expand or enlarge without adding any new portion of matter to.

Distant from the lives and everyday concerns of ordinary people; esoteric, exclusive, select. / Elevated in style or nature, sublime; of high intellectual or moral value.

simple past tense and past participle of rarefy

Welsh rarebit