The "half domes" on these brooches are either 18 mm rounds, 18 X 25 mm ovals, or 30 mm donuts. In most every picture you will see two images of each brooch. The left image shows the brooch with antique brass plating and the right image shows the brooch with antique nickel plating. If both types of plating are pictured, you must specify antique brass or antique nickel plating to complete your purchase.