This happens during the production process. The main reason is that the module is too small for stamping 925 or 990. What is Sterling Silver .
5 Sterling Silver Oval Jump Rings 925 Silver Open or Closed Bracelet Connector
Sterling Silver Oval Jump Rings 925 Open 3 4 5 6 7 8 9 10 mm- UK QUALITY
50 Sterling Silver Open Jump Rings 925 Silver Closed Jump Ring 3mm 4mm 5mm 6mm
6 Sterling Silver Circle Jump Rings 925 Silver Wired Large Hole Bead Spacer
925 Sterling Silver Oval Jump Rings 4mm 5mm 6mm 8mm 10mm Jewellery Making
5pcs of 925 Sterling Silver Oval Jump Rings Connectors for Necklace Bracelet
