Yes, MATRICES is a Scrabble word!

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

Is it a valid word?

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

(mathematics) a rectangular array of quantities or expressions set out by rows and columns; treated as a single element and manipulated according to rules
(geology) amass of fine-grained rock in which fossils, crystals, or gems are embedded
an enclosure within which something originates or develops (from the Latin for womb)
the body substance in which tissue cells are embedded
the formative tissue at the base of a nail
mold used in the production of phonograph records, type, or other relief surface

Also: intercellular substance, ground substance.