Gratonite A rarity only found in appreciable size in one place on Earth (the Excelsior mine at Cerro de Pasco in Peru), it is a sulphide of lead and arsenic, hence the grey colour and metallic lustre. Needless to say the mineral is toxic, so basic precautions like handwashing after handling and not licking are advised. First noted in 1939, it was named after a distinguished professor of mining geology at Harvard with a long relationship with the mine, and as you might guess it is both dense (6.3 times more than water) and soft (2.5 on Mohs scale). Its typical habit is as clusters of rhombohedral crystals like the 2.2 x 1.4 x 1.1 cm (large for the species) specimen in the photo. It is in an odd category of minerals known as sulphosalts...indeed they often contain the elements known as metalloids such as arsenic, bismuth and antimony that can act as a metal or not, depending on chemical circumstance. In these cases the arsenic is acting metallic. Minor occurrences are found in Cuba, Poland, Bulgaria, Germany, Switzerland, the Isle of Man, Spain, Namibia, and Japan. The exceptional piece featured is historic, mine in the late 19th century CE. Loz Image credit: Rob lavinsky/iRocks.com https://www.mindat.org/min-1741.html http://www.galleries.com/Gratonite