I hand cut this cabochon from native copper ore from the Keweenaw Peninsula in Michigan's copper country. It is a mixture of solid native copper in matrix. Measurements in the title in millimetres. This material does oxidize or tarnish as all natural metals do but the shiny copper finish can be quickly returned with a simple rub with a polishing cloth. I do NOT put a layer of acrylic over my natural metal mineral cabochons to prevent tarnishing as some dealers do as I feel that a plastic sealant ruins the experience of the stone. See ALL my work with custom cut focal beads, cabochons, druzys, mourning jewelry, fordite, chains, and much more in my DVHdesigns store.