part of my lockdown research project is determining bead materials -currently online.
looking online is seldom reassuring because so many collectors and dealers too readily believe and repeat what they have been told, even if they aren't exactly trying to fool anyone.
now i'm looking at shell beads particularly from the himalayan areas, and find the differing terms are used-
Tridacna is a very pure large white bivalve clam-type thick shell, not spiral, with big wavy fan-like ridges. in China today sets of matching smooth round beads are made and sold at high prices if the colour is perfectly white. i was given a 108 bead strand some years ago. while they are definitely shell material their uniformity is rather boring, and they don't have the lustre of Mother-of-pearl.
i discover that Chank is the name in India for the shell otherwise called Conch. i'm sure Jamey will be able to provide more information. its a name for the large spiral shells that are cut into discs, long fusiform shapes or rounded scoop shapes that feature in Naga necklaces and girdles, and sometimes have designs of incised dots. the long narrow beads are shaped from the spiral central column shape. And some copies these days are made from porcelain...
One question is how do so many venerably large old shells reach the high mountains- and also seem to be the material of many large prayer strands,(even being a tibetan buddhist symbol), where the spiral structure is visible- specially if they are also called Tridacna?
I may be able to sort out a few images by the weekend-