Ok! Here is the answer: (I added another image that makes things clearer) it's a simple drawing paper, only blue, with tons of children's glitter! We were painting with brushes with my son and he spotted the little jars with glitter in his cupboard -the cheapest material you can imagine. So he took the blue paper, emptied all four little jars on it and mixed everything together with the brush. The smell was so intense I took it out on the balcony 'to dry' I told him, 'to throw away at some point' I thought. I couldn't believe my eyes the following morning when it was all dry and 3-dimentional from the huge amount of paint on it! I'm planning to frame it and put it up in his room.
Thanks for playing along ladies! Some good guesses there!
PS. Note to myself: buy some wooden pegs!