They look like died nucleated freshwater pearls to me. 'The type is called 'fireballs' sometimes...
I am not sure whether the oil-on-water colors on such pearls are orient only, in some cases there seems to be an excess of oxidized die that also has waves of rainbow colors on its own only of somewhat different appearance: static, more intense but flat... not sure how to put it, but it is an instantly recognizable look once you've seen any large spot of dried printing ink.
What makes them this way... I've asked that at some point (another thread around here) and the answer came that #1. the anatomy of freshwater mollusks does not allow for the pearl sacks around implanted nuclei to remain in a steady shape, so the result are mostly baroque pearls as layers of nacre keep changing shape with the sack, and #2 that the 'tails' are formed because of slow closing pearl sacks bulging on one side of the the nucleus - something that can and does happen with some baroque saltwater nucleated pearls too, so there is some similarity between the types (which makes good PR for freshwater fireballs). I tend to believe the stories of pearl sacks. A search for 'fireball pearls' might bring those threads up too.
There were a couple of strands like yours discussed here already, but there doesn't seem to be that many in general.