My vote still goes for bone, although I guess, it theoretically could be from the outer layers of the clamshell, where the material is more mottled and of lesser quality. In my experience though, craftsmen usually take from nearer the center, where the shell is harder and more compact.
An example would be these Japanese Go-stones, also carved from some unknown
Tridacna-species:
- and yes, I am aware that these pictured, are expensive über high-end stones, but they serve to illustrate that purity is an ideal when it comes to worked clamshell.
Rick's suggestion is good IMO. If the handle feels hard, cold and stone-like to the touch, it's probably Tridacna-shell. If you could carefully burn some unexposed part of the hilt, you would quickly be able to determine whether the material is organic or mineral.
*Wouter* I agree with you, but I'm sceptical about the source being caribou though. Unless of course you're referring to the epic 18th century Moro-raid on Santa's Sleigh? It was a massacre... Antler, red noses and little elf limbs everywhere...
Don't you think it's more likely to be from sambar deer or something likewise more local?
Best wishes, - Thor