Pad and brooch component are both 20mm. 5, 10 or 20 Silver Tone Round Brooch Backs. (As with most brooch backs these are not suitable for soldering to but can be glued).
The "Round Brooch Backs 20mm Silver Tone Pins Blanks Badge Broache" product would be a suitable fit for jewelry professionals looking for high-quality, elegant blanks to create custom brooches and broaches with.
