Our research has led us to believe that this micro mosaic picture is St. Peters Basilica. Its overall measurements are 1 15/16" by 1 1/2" and is 3/16" thick. We believe it to be set in black onyx. We cannot say for sure its age or if it was intended to be a brooch etc. It shows some nibble marks around the edge and a couple of missing pieces but overall it is in excellent condition. I will be happy to send further photos if there is a specific angle or shot you would like to see.