3 Netsuke and a dragon head stamp -any idea what they could be made of?

I think these are so neat, but would like to know more about them. I was given these as a gift, but have no idea what they are made of or how old they could be, they failed the amber tests, and I cant think that they are plastic -no seams , all but 1 are signed and the work on them is so fine and intricate! Anyone out there that could help identify these? Many Thanks