Spalting (not sure I'm spelling it correctly and too lazy to do a simple web check) is a fungus that gets in the tree and leaves a pattern similar to what is in those grips. It is very common, desirable, and sought after in maple. Being a fungus and semi-rot, you stabilize it by filling with cyanoacrylate glue (Super Glue, Krazy Glue, Aron Alpha, etc.).