Bruising is when the crystals in the stone (which is what makes them so pretty) get crushed by a too-direct blow of a chisel. They can be dug out using a riffler but this may make you change the shape of the stone. To prevent this NEVER hit the chisel into the stone at a steep angle. About 45 degrees seems to be about right but it changes with the hardness of the stone. The harder the stone the steeper the angle required, and the more care that is needed. Marble can be bruised quite deep. Alabaster can be brusied but is easier to file out.