This sponge fossil is called Porosphaera globularis from the Cretaceous Period. They were found in the region of Navarra, Spain.
According to the Natural History Museum of the U.K, people sometimes use these fossils as beads in necklaces. One was even found at a Bronze Age burial site with a skeleton from the Higham Marshes of Kent in south east England.
Thanks to Herb for these fossils.
Info sources:
http://uk.naturalhistory.museum/nature-online/earth/fossils/fossil-folklore/themes/decoration06.htm
http://www.thefossilforum.com/index.php?/gallery/image/29192-porosphaera-globularis/
