Thank you magicsound. Very useful analysis. Have you tried an oxidized region as can and Storms have suggested?
Nope, but it's possible to color the surface with a gas torch. That creates a very thin layer of oxide though. The color is only visible near the angle of light incidence, suggesting the oxide layer thickness is around the wavelength of visible light. It doesn't change much from additional heating once formed, and it leaves the Ni dead soft, almost like Al foil, making depositing any Pd from burnishing unlikely.
The most common form Ni[II]O is the form in nature from which Ni metal is refined. Here's a quote from https://chem.libretexts.org/Bo…etals/Chemistry_of_Nickel
"Nickel oxide is a powdery green solid that becomes yellow on heating. It is difficult to prepare this compound by simply heating nickel in oxygen and it is more conveniently obtained by heating nickel hydroxide, carbonate or nitrate. Nickel oxide is readily soluble in acids but insoluble in hot and cold water."
There's also "Nickel(III) oxide Ni2O3. It is not well characterised, and is sometimes referred to as black nickel oxide." (Wiki).