The first heart was tatted with Altin Basak thread and the center is made from a stone called Rhodochrosite. The second heart was made with Manuela thread and the center is made from bone. The third heart is also made with Manuela thread and I'm not sure what this center of this is called.
These Hearts were designed by Jane Eborall and you can find more exciting patterns by Jane on her web page. Just click on her name.