What do you call this technique where you give the metal surface a rugged/aged look like the star in this picture? Is there a link on how to do that? Can I finish-seal it as a protection?

I'm in school for metal fabrication and this is going to be my first project (for a female friend of course). It'll look good in my my portfolio too.
Many thanks.
I'm in school for metal fabrication and this is going to be my first project (for a female friend of course). It'll look good in my my portfolio too.
Many thanks.
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