Phosgenite A rare lead carbonate with chlorine added in, it shares with other lead minerals a heavy density (6.3 times denser than water) and a high lustre (surface reflectivity) nearly as bright as diamond's. While it is usually colourless yellow, brown, greenish or pink material has also turned up in various places. It was named in 1820 after phosgene subsequent to its first discovery in the lead mines of Derbyshire in England (it is also called Comfordite after its type location). It is brittle and soft (Mohs hardness of 3) and so almost never faceted. It forms in the oxidation zone of lead deposits where chlorine and carbonate rich waters (such as sea water or deep Earth brines) have altered the primary oxygen poor minerals into secondary ones. It exhibits the property of fluorescence in UV light, whereby electrons are excited by the energy and jump up a level before releasing the energy as they form back down as visible light. Other localities include Sardinia (whence hails this lovely gemmy 3.0 x 3.0 x 2.2 cm crystal and most of the larger specimens on the market), Greece, Poland, Siberia, the USA, Namibia and Tasmania. One intriguing locale has been in ancient Greek slag dumps from mine ore processing that were dumped into the sea. Loz Image credit: Rob Lavinsky/iRocks.com https://www.mindat.org/min-3195.html http://www.galleries.com/Phosgenite http://www.minerals.net/mineral/phosgenite.aspx http://bit.ly/2Ac32Ip http://bit.ly/2iThnSZ http://bit.ly/2C5MMdl