Yes, PERIOSTEA is a Scrabble word!

Letter values in Scrabble US: P 3, E 1, R 1, I 1, O 1, S 1, T 1, E 1, A 1.

Is it a valid word?

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

a dense fibrous membrane covering the surface of bones (except at their extremities) and serving as an attachment for tendons and muscles; contains nerves and blood vessels that nourish the enclosed bone