I have started sculpting a new master that features a keratin sheath. Keratin is the protein that hair and fingernails are made of, and all animals with exposed claws have a keratin sheath that protects the bone. The black line denotes where the sheath stops, and the flesh of the toe begins. Because the section of bone directly under the keratin is more dry, it also dries and cracks faster during the fossilization process, which informs the cracking patterns in the fossilized claws seen in this thread.
Above is actually version 2 of this project. I previously produced another raptor claw with a keratin sheath (seen below). Version 2 will be an improvement. I'm hoping to overmold this claw, with the exposed bone being white resin, and the keratin sheath being dyed resin.