This is a copy of one of the 11thC Anglo Saxon finds from near Bow Church on Cheapside. The originals are in the Museum of London.
The back is a little speculative, as the original only had a pair of posts on the back, with no pin, and I've not been able to examine them closely. Other finds in the hoard have remenants of soft iron wire pins fitted to the posts.
The red glass stone is cast in place, just like the originals.
Your brooch is likely to contain some tiny voids as casting flaws, Just like the majority of the originals. Period processes lead to period results.