Where is nickel in the rock cycle?

Nickel is a metallic element. It is not found as a pure metal on the Earth's surface. There is however a vast quantity of metallic Nickel present in the Earth's core, it sank to the core along with all the metallic Iron on Earth and much of Earth's heavy elements as part of the 'Iron Cascade' (see related "planetary differentiation" link below) when the whole planet was molten.

The Nickel found as ores in surface rocks today was delivered to the earth by meteorite impacts after the Earth's surface had solidified. All the heavy elements found in surface rocks were delivered in this late, secondary veneer of planetary accretion.