rare

Marked by an uncommon quality; especially superlative or extreme of its kind.

“what is so rare as a day in June”

“a rare skill”

Having low density.

“rare gasses”

“lightheaded from the rarefied mountain air”

Not widely distributed.

“rare herbs”

“rare patches of green in the desert”

Not widely known; especially valued for its uncommonness.

“a rare word”

“rare books”

(of meat) cooked a short time; still red inside.

“rare roast beef”

Recurring only at long intervals.

“a rare appearance”

“total eclipses are rare events”