Copper - an important ore and also a gemstone (sometimes)
I wrote book about copper deposits in Wyoming and it led me to the conclusion there are many copper occurrences and dozens upon dozens of cupriferous minerals and cupriferous ores. But Wyoming is not unique: when one examines many of western states such as Alaska, Arizona, Colorado (be careful in Colorado - some of those people can find just about anything after a few joints), Idaho, Montana, Nevada, New Mexico, Utah and Wyoming, you can spend a lifetime trying to collect all of the copper minerals. But the west is not unique, as many copper deposits have been found across Canada, Mexico and the United States. In some places, there is such much copper, one could decorate a house or even a town with all of the copper. And for a lot of people, its hard to imagine so much copper and other metals and gemstones can be sitting right on the surface and much of it untouched and overlooked by others.
Many copper-bearing minerals have been used as semi-precious gemstones, but most are soft and must be protected. Others produce spectacular collector specimens, decorative stones and lapidary. There are so many copper minerals that when I was in graduate school, one of my office mates built a reputation and PhD dissertation on identifying previously undocumented copper minerals.
In many of the Western States, the more one searches the mountain sides, the more copper minerals they will find, but in the copper state of Arizona - copper is literally everywhere! You will find it in the hills, basins, peoples yards and even in some parking lots. The photo in the upper left is a boulder of azurite (blue), tenorite (black), dark red (cuprite) and minor malachite (green) from the Morenci, Arizona district. Imagine wearing that boulder as a pendant around your neck.
Although azurite (H=3.5 to 4), malachite (H=3.5 to 4), chrysolla (H=2.5 to 3.5) and tenorite (H=3 to 4) can produce extraordinarily specimens due to vivid color, their poor hardness precludes much use of these as gems because they are so easily scratched. Thus these and other soft copper minerals must be protected. Others such as turquoise (H=5-7) are more favorable for wear.
One district characteristic of most copper minerals is a reaction to dilute (10%) hydrochloric acid. Copper carbonates such as azurite and malachite will fizz when they react to hydrochloric acid, while most copper minerals will replace rusted iron with native copper. Geologists pack around old beat up rock hammers and place a few drops of weak hydrochloric acid on a mineral that is suspected to contain copper. Next they will rub their rock hammer in the wetted mineral. After it turns to mud on the rock hammer, they clean off the mud. If the mineral contains copper, any rusty scratches in the rock hammer will be replaced by native copper.
(1) Hausel (1997), The Geology of Wyoming's Copper, Lead, Zinc, Molybdenum and Associated Metal Deposits in Wyoming:Geological Survey of Wyoming Bulletin 70, 224 p., and
Hausel (2009), Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.
Hausel, 2014, A Guide to Finding Gemstones, Gold, Minerals and Rocks: Gemhunter Books, 367 p.
Photo show a specimen with covellite (copper sulfide) from Butte Montana.