Yes, SQUARK is a Scrabble word!

Letter values in Scrabble US: S 1, Q 10, U 1, A 1, R 1, K 5.

Is it a valid word?

SQUARK is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

a quark with an electric charge of -1/3 and a mass 988 times that of an electron and a strangeness of -1

Also: strange quark.