*David* Thank you for drawing my attention to your link - again. I had indeed missed it the first time around. I see now, that the mottled parts of the shell are indeed being used - at least when we're talking keris hilts. I just find this peculiar. I have myself manually destroyed dozens of
Tridacna-shells and even fairly small half-shells, say the size of your average popcorn bowl have maybe 6-7 cm thick shells. Especially the area near the
umbo (hinge) of the clam, is comprised of plenty of dense, white material.
Atm. I tend to agree with Wouter: I'm still indecisive, but after seing the phtographs Rick linked to, I'm leaning more towards clam than bone. It's not however TOK-TOK. Bone against glass goes PLOK-PLOK, while clamshell against glass goes KLINK-KLINK.

The problem with onomatopoeia is the same as with the hot/cold-distinction: They're both very subjective..
I still say torch it - it's the only way to know for sure! If dissassembly is not an option, I guess comparing it with the sound of a stone against glass, is the next-best thing: They should sound about the same: both harder than bone.
- Very cool discussion. I'm very grateful to have learned this and seen the above photos. Thanks everybody!
All the best, - Thor