"Diamond" brooch

I love this brooch, but really know nothing about it. Just one more thing I found out walking my dogs. Just one little corner sticking out of the dirt caught my eyes and I carefully dug it up. I couldn't believe it when I realized that only one stone was missing! There is no mark on the piece so I don't know who made it, where it was made, how old it is, or what it's made out of.
Note: I took one pic with the brooch hanging from a chain and using the flash.