Double pseudomorphs are very rare in nature, and I think most collectors would rank these rare old USA classics among the best and most beautiful of them, worldwide. This formed after the originally-formed azurite crystals were later changed to malachite which, in turn, became chrysocolla. Thus we have the azurite crystal shape with chrysocolla chemistry, now - and the beauty of a deep robin's-egg blue color. Truly, they are a unique thing caused by a very unusual combination of events at this mine. This is an old USA classic, but seldom seen today on the market. I buy every one I see! This has a very large crystal, unusually robust and long, measuring 5 cm across. That is the largest such crystal I have personally seen. Ex Conan Barker collection.