Potosiite is the rare Tin poor variety of the rare mineral Franckeite, and was a separate species until it was discredited in 1997. It is still a very rare Sulfide with crystals larger than micros considered huge. From a small find of the worlds largest and best crystals at the San Jose mine, this is a top example with individual crystals to 1.3 cm. For this rare and ugly mineral this is a true killer and even esthetic! This was rated as one of the best 7 pieces found.