Yes, METASTABLE is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, T 1, A 1, S 1, T 1, A 1, B 3, L 1, E 1.

Is it a valid word?

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

(of physical systems) continuing in its present state of equilibrium unless sufficiently disturbed to pass to a more stable state of equilibrium