These are a pair of large showy dress or cloak clips. I have researched the crown mark on reverse side and likely that these are by Brier manfacturing, Rhode Island, a prolific jewellery company in the 30’s and onward era.
The spring mechanism has a fancy design, which is something this company always did in case the underside was viewed while wearing.
Each dress clip measures 2.5” end to end and 2” across.
I think they would easily convert to a pendant with the addition of a chain and look fantastic worn like that.