What is the difference between EMAX, Zirconium, and Procera?

Hi, I have had a couple of different quotes to have crowns put on for cosmetic dentistry 12 (6 upper and 6 lower). Both quotes are about the same price but different materials. One quote is for EMAX and one is for Zirconium. I also had another quote for Procera which was even more money. Can you advise me on the difference between the materials used for crowns. I want to make an educated decision on which dentist to choose.