Nickel ore is a crafting component introduced in the Blood and Wine expansion that can be dismantled into hunk of nickel. It is not needed to craft anything.
Crafting Requirements 
It can be crafted, found as random loot, or purchased from the following merchants:
- Armorer in Hauteville
- Blacksmith at Ardaiso Quarry
- Blacksmith in Belgaard Manor, after he is rescued
- Lazare Lafargue in his workshop
- Trentin in his workshop in Hauteville