Zirconium, Zr, is element 40. It is a transition metal. Information about it is easy to come by, so perhaps this article should only mention a couple of curious points about Zr.
a. Zr forms a dioxide, ZrO2, known as zirconia. Crystals of zirconia are called zircons, and they are very stable mechanically and thermally. In their impurities, they carry a record of the time when they formed. One such time was the forming earth. Zircons are the oldest minerals we've found.
b. Zr is a good material to use to clad nuclear fuel pellets because of its low neutron-capture cross section. Hafnium is a good material to use for reactor control rods because of its high neutron-capture cross section. Chemically, Zr and Hf are among the most difficult elements to separate. Another little prank of nature.