Such clam shell rings are called 'wenga', and used by the Boiken people of Papua New Guinea as means of payment. There exists no common scale of valuation; the Boiken people rate them for individual considerations. Size matters, of course – the bigger, the more valuable. On the other hand the rings are higher in value the narrower they are. This has to do with the danger of breaking them while drilling the hole.
Wenga are made from the shells of the giant clam Tridacna gigas. It is the largest of all known clam types, with lengths up to 140 cm and weights up to 500 kg. The shells are cut into disks, a hole is drilled into each disk and then the ring can be polished.