Yuki (雪, snow) is the Japanese go term for the grade of shell stones that many consider the perfect White stones. Shell stones as a class are preferred to glass or plastic.
Yuki will have the grain run straight across the stones without a bend.
Lower grades of stones include tsuki (curved grain on stones) and jitsuyo (which have an indistinct grain).
Important in the aesthetics of Go.
-- Tim Brent
Yuki grade size 30 stones
Single yuki stone