Yes, CONSTANCY is a Scrabble word!

Letter values in Scrabble US: C 3, O 1, N 1, S 1, T 1, A 1, N 1, C 3, Y 4.

Is it a valid word?

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

the quality of being enduring and free from change or variation
(psychology) the tendency for perceived objects to give rise to very similar perceptual experiences in spite of wide variations in the conditions of observation
faithfulness and dependability in personal attachments (especially sexual fidelity)

Also: perceptual constancy.