This peculiarity appears to be caused by the thinness and semitransparency of the supposed layers.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
They quickly lose their peculiar glistening polish, their semitransparency is lost, and the natural tint of a pearl-like blue gives way to a dirty yellow.
"Diseases of the Horse's Foot"
Harry Caulton Reeks