There are very similar shells forms clams, etc, all over the edges of Bremerton Island a little outside Seattle where my brother used to live.I agree, it's opal. Virtually all opal found in the state is common opal. Excellent find, and it would be called a white precious opal, because of the variety of color it has. Find that gravel pit!
One question. Is your driveway crushed rock, or river-run? That will point you toward the right gravel pit or rock quarry.