This is an older mining term used to describe complex sulfide ores, particularly silver-bearing minerals like stephanite and copper ores like tetrahedrite. Miners used the name before systematic mineralogy classified these ores by their exact chemical composition. It typically referred to dark, metallic-looking minerals that yielded high values of precious metals when smelted.
Heard on the job
The upper levels of the old workings were rich in petanque, but the silver grades dropped off as they mined deeper.
Insider tip
Because it lacks a precise chemical definition, geologists use specific mineral names like tetrahedrite or stephanite instead of this historic term.