Maybe there are better, but I do not know of them. This is the largest and best crystallized natural chalcanthite that I have personally seen. It has a huge complete crystal of neon blue, sticking straight up from matrix. It is pristine, dramatic, glows when backlit, and is from an old-time and romantic locality. Bottom line, though, it is just "pretty!" Chalcanthite is easily synthesized today and grown in labs all around the world for the gift shop and fake rock markets. Chalcanthite forms as post mining or manmade crystals from working copper mines such as in Arizona. It can be bought in the Smithsonian gift shop and grown from powder on the kitchen table. However, aside from the chance that this was post-mining (and I do not think it was), my bet is they were not making these in labs and from gift shop chemistry sets back in the early 1900s in Chile. With as many copper mines as we have around, I have not seen any post-mining chalcanthite formed from the runoff of human workings, that reaches this magnitude. So I find it unlikely this was formed in that manner, although its a slim outside chance. This is, surely then, natural as the museum itself concluded in the end: certainly one of the most dramatic examples of the species I can imagine. The wooden display label with it indicates exhibition in a case of trophies on return from one of the famous Vaux expeditions of the 1920s, apparently. Note another, clearly post-mining (but naturally made) American chalcanthite of substantial size in this update (PAS-178), as well.
Ex. Philadelphia Academy of Natural Sciences