Where in the world did this come from??? Is it a pendant?

Always wondered where this came from. I found this 60 years ago on the street, and it always intrigued me. Maybe I was meant to be a geologist:) Anyway, I think the background is brass. I see no markings on this and lightened it up, both front and back for a closer look. The stones are iridescent and are cut in a diamond cut fashion, so they reflect the light. It is 2 1/4" across. The chain is not the
original one. Speaking of light, if anyone can shed some light on the origin of this I would be most appreciative!