There is so much going on with this super-rich specimen from the classic Zacatecas mines. The iridescent blue color is covellite (copper sulfide), and the darker stuff is bornite (copper iron sulfide). But on this same specimen, you see both massive and crystallized silver, here and there in the form of fine wires. This old-time specimen is ample demonstration as to why the Zacatecas mines were so productive and valuable.