Yes, CERIUMS is a Scrabble word!

Letter values in Scrabble US: C 3, E 1, R 1, I 1, U 1, M 3, S 1.

Is it a valid word?

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

a ductile grey metallic element of the lanthanide series; used in lighter flints; the most abundant of the rare-earth group

Also: atomic number 58.

Not playable in Scrabble US: CE.