Nickel is an element, atomic number 28 on the 21st century periodic table.
The water of a Class M planet surveyed by Nog and Jake Sisko in 2370 contained traces of nickel, copper and birythium. (DS9: "The Jem'Hadar")
Many planetary cores are composed of an iron-nickel alloy, either in solid or liquid form.
The planetoid in the Gamma Canaris region where Zefram Cochrane lived with the Companion is composed of iron and nickel. (TOS: "Metamorphosis")
A M-class planet located in the Ngame Nebula that was terraformed by the Paxans and discovered by the USS Enterprise-D had a nickel-iron core. (TNG: "Clues")
Sensor scans of the asteroids artificially constructed by the Etanians, used to attack a Nezu colony, were composed of a simple nickel-iron composite. (VOY: "Rise")
A planet that the Delta Flyer crashed on in 2375 had a Class M atmosphere, nickel-iron core, and bemonite mantle. (VOY: "Once Upon a Time")