Proustite
Proustite today is a highly valuable rare collector's mineral, especially when in red color & transparent. It's an interesting mineral that contains silver in its chemical structure, one of the few silver-bearing minerals that can exhibit transparency. Usually transparent, with deep-red crystals, but it may also be a darker, more metallic-looking form - though even darker, more metallic versions will be visibly red & transparent when back-lit. Best examples are from Chañarcillo, Chile
