Shell is the calcareous exoskeleton of molluscs.  These animals secrete shell from their mantle for protection and support.  Shell is made up of calcium carbonate and/or aragonite crystals which bind tightly together but can cleave along layers.  Shell can be flaked by percussion but tends to fracture along the cleavage planes, preventing the removal of long flakes.  Shell was often used as a substitute for stone, particularly in the Pacific.  Most shell working was done by grinding although percussion flaking was sometimes done in early manufacturing stages.