Actually crab claw tips and Otter "fingers" have been covered with
nacre to form pearls. Often it is a piece of wood, wayward kelp
piece, etc. Some ab pearls are many and small roundish objects, most
likely sand. Doubt if shark teeth, but crab claw tips are surely a
potential. Also, most ab pearls sold on the market are shell pearls,
"pearls" cut from the inside of the shell. These tend to be formed
in areas where tube worms drill all the way through the shell and the
hole on the inside is then covered with nacre forming a "bump" in the
shell that is cut out and sold as a "pearl". The ones we have been
talking about and the posted pictures, were "gut pearls" objects that
got into the animals gut and then were covered with nacre. Shell
pearls most often are "colored" or coated with nacre on one side, the
side facing into the gut/foot area. Gut pearls are formed in the gut
and are coated on all sides with nacre.
Probably more that you wanted to know!!
Lastly, there is a small industry in parts of the world making the
shell pearls that are sold through wholesale/retail outlets, but I
have never seen gut pearls sold from these sources as they are so
VERY rare, much more so than natural oyster pearls.