Yes, CORNERSTONE is a Scrabble word!

Letter values in Scrabble US: C 3, O 1, R 1, N 1, E 1, R 1, S 1, T 1, O 1, N 1, E 1.

Is it a valid word?

CORNERSTONE is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the fundamental assumptions from which something is begun or developed or calculated or explained
a stone in the exterior of a large and important building; usually carved with a date and laid with appropriate ceremonies
a stone at the outer corner of two intersecting masonry walls