Noteworthy scarcity.

synonyms: infrequency, rareness.

Roget 103: fewness etc. adj.; paucity, small number; small quantity etc. 32; rarity; infrequency etc. 137; handful, maniple; minority; exiguity.    [Diminution of number] reduction; weeding etc. v.; elimination, sarculation^†, decimation; eradication.    ... show more

Roget 137: infrequency, rareness, rarity; fewness etc. 103; seldomness^†; uncommonness.   

Dutch: zeldzaamheid
Polish: unikat, rzadkość, wyjątek, ewenement

A rarified quality.

synonyms: low density, tenuity.

Roget 322: rarity, tenuity; absence of solidity etc. 321; subtility^†; subtilty^†, subtlety; sponginess, compressibility.    rarefaction, expansion, dilatation, inflation, subtilization^†.    vaporization, evaporation, diffusion, gassification^†.    ether etc. (gas) 334.    ... show more

Something unusual -- perhaps worthy of collecting.

synonyms: curio, curiosity, oddity, oddment, peculiarity.

Roget 83: nonconformity etc. 82; unconformity, disconformity; unconventionality, informality, abnormity^†, abnormality, anomaly; anomalousness etc. adj.^†; exception, peculiarity; infraction of law, breach, of law, violation of law, violation of custom, violation of usage, infringement of law, infringement of custom, infringement of usage; teratism^†, eccentricity, bizarrerie^†, oddity, je ne sais quoi [Fr.], monster, monstrosity, rarity; freak, freak of Nature, weirdo, mutant; rouser, snorter [U.S.].    individuality, idiosyncrasy, originality, mannerism.    aberration; irregularity; variety; singularity; exemption; salvo etc. (qualification) 469.    nonconformist; nondescript, character, original, nonesuch, nonsuch^†, monster, prodigy, wonder, miracle, curiosity, flying fish, black sheep, black swan, lusus naturae [Lat.], rara avis [Lat.], queer fish; mongrel, random breed; half-caste, half-blood, half-breed; metis [Lat.], crossbreed, hybrid, mule, hinny, mulatto; tertium quid [Lat.], hermaphrodite.    [Mythical animals] phoenix, chimera, hydra, sphinx, minotaur; griffin, griffon; centaur; saggittary^†; kraken, cockatrice, wyvern, roc, dragon, sea serpent; mermaid, merman, merfolk^†; unicorn; Cyclops, men whose heads do grow beneath their men whose heads do grow beneath their shoulders" [Othello]; teratology.    [unconformable to the surroundings] fish out of water; neither one thing nor another, neither fish nor fowl, neither fish flesh nor fowl nor good red neither fish flesh nor fowl nor good red herring; one in a million, one in a way, one in a thousand; outcast, outlaw; off the beaten track; oasis.    ... show more

Dutch: curiositeit, curiosum, rara avis, rariteit, zeldzaamheid
Polish: cacko, cudo