"A livelier ____ twinkles in the grass, / A purer sapphire melts into the sea” (Tennyson, in Maud) (7)
I believe the answer is:
emerald
(Other definitions for emerald that I've seen before include "Green like this isle" , "Shade of green" , "and 13 Across: Poetic name for Ireland" , "and 1 Across: Ireland" , "Bright green colour" .)