how can i oxidise a jewelry?

Answer 1

You realize you're essentially asking "how can I take something valuable and destroy it," right?

If it's copper or silver, have it sit around, or in a slightly acidic environment, e.g. vinegar.

For any jewlery, including gold and platinum, use aqua regia.
http://en.wikipedia.org/wiki/Aqua_regia

That'll oxidize any metal, and as a bonus, probably kill you in the process. It's *extremely* nasty stuff.

Answer 2

Expose it to oxygen.