Somebody on our sister site (antiquers.com) aked what this is. It looks to me like it could be an ancient coin, but I'm not sure.
I don't think it's a button. It looks like it was drilled to wear on a chain. But what do I know? LOL
I don't think it's an ancient coin... it appears to be some kind of pendant for a necklace. Maybe you'll be able to find out more if the encrustation is cleaned off. It looks to be copper...
That’s certainly not an ancient coin. My first guess would be that you have some sort of appliqué, maybe from a belt or from horse tack. Old saddles sometimes were decorated with metal conchos similar to your item. A piece of decorative furniture fitting might be another possibility.
@Theodosius, that was my first guess (x2), but it doesn't look like a coin. The findspot sort of clinched it.... Am I making it up, or is there a little bit of concavity?