To make sure we get what we have bought and there are no harmfull materials in de jewerly we are buying, we can ask our suppliers to provide us with theil mill certificates. A lot of suppliers put them on their website aswell for transparency reasons.
A mill certificate is a document provided by manufacturers or suppliers of metal materials used in the production of body jewelry. These vertificates provide detailed information about the composition, quality and characteristics of the metal used to make the jewelry. On these certificates, you can normally find what alloy has been used, if it is compliant with ASTM or ISO standards, the origin of the materials used and sometimes the surface finish of the jewelry.
An example below: you will find in this mill certificate that the seller of the jewelry (and the companie who asked for the certificate) is Industrial strength.