Yes, RHOMBOIDS is a Scrabble word!

Letter values in Scrabble US: R 1, H 4, O 1, M 3, B 3, O 1, I 1, D 2, S 1.

Is it a valid word?

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

a parallelogram with adjacent sides of unequal lengths; an oblique-angled parallelogram with only the opposite sides equal
any of several muscles of the upper back that help move the shoulder blade

Also: rhomboid muscle.