While selecting stone suitable for flag (1/2"" to 2 1/2"" thick), employee encountered a stone which was too big on one end (2 1/2' x 1 1/2' - 2"" on one end & 4"" on the other end). He dropped it on a stone ledge to break larger end off. This caused 1 of several fist size pieces to fly up & hit him in the eye, knocking his safety glasses off.