Yes, AG is a Scrabble word!

Letter values in Scrabble US: A 1, G 2.

Is it a valid word?

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

a soft white precious univalent metallic element having the highest electrical and thermal conductivity of any metal; occurs in argentite and in free form; used in coins and jewelry and tableware and photography

Also: atomic number 47.