Yes, DIAGONALS is a Scrabble word!

Letter values in Scrabble US: D 2, I 1, A 1, G 2, O 1, N 1, A 1, L 1, S 1.

Is it a valid word?

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

(geometry) a straight line connecting any two vertices of a polygon that are not adjacent
a line or cut across a fabric that is not at right angles to a side of the fabric
an oblique line of squares of the same color on a checkerboard
(mathematics) a set of entries in a square matrix running diagonally either from the upper left to lower right entry or running from the upper right to lower left entry
a punctuation mark (/) used to separate related items of information

Not playable in Scrabble US: SEPARATRIX.