Any Cleaning Gurus out There?

[Capncrunch]

Hey all, I found a really cool-looking hatpin (at least the original condition looked cool), today. It is covered with a hard (like concrete) substance that is plain to see in the photos. IDK what it is plated with (probably fake gold?) but would love some suggestions on how best to clean it, without damaging it any more than it already is!
Anyone here ever clean a similar item successfully, and if so, what did you use?
Thanks!

 

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