For now I just made the pin test( not hot as it needs more time and concentration that I don't have currently )and it's not hollow at all. Interestingly the pin goes through the material without effort just pushing a bit inside, whatever it is.
I notice too that the large hole I show in photo is only on one side, the other side it is very tiny. So drilled ?
Thank you birdi, I didn't know about cotton ball beads. I'll have to study about with more time and the style of the necklace looks to me yes, japanese too... Thanks for your suggestions !