A glass eye...can also be referred to as a watch eye...they are blue....with what looks like a small iris...almost like a human...the blue of the eye is surrounded by white....you see them in horses of lighter pigmantation...I.E. appaloosas'...any albino horse.....

chestnuts also get them alot......

they basically look like a blue eye on a husky dog or an australian shepherd.....

hence the name:
Glass Eye