Black Hills rose quartz

Rose quartz has been South Dakota's state mineral since 1966, adopted alongside the Fairburn agate as state gemstone, and it comes from the old crystalline heart of the Black Hills. The southern Hills are Precambrian granite, pegmatite and metamorphic rock, the core of a domed uplift ringed by younger sedimentary layers like a target. Around the Harney Peak granite, now Black Elk Peak at 7,242 feet, masses of pegmatite push through the schists, some of them miles from the granite, and those near Keystone hold the greatest number of commercial deposits. The rose quartz deposits near Custer are the other classic example.

The best known of them is the Scott Rose Quartz mine, southeast of Custer, which the US Bureau of Mines recorded as a surface producer of silica, with feldspar and mica, on National Forest land. The Mineralogical Society of America's guide to the region lists it among the pegmatites deserving special note, with the Beryl Mica Lode, Climax, Wood Tin, New York and others.

The pegmatites around Harney Peak have yielded several rare minerals, some of them new species, and minerals of extraordinary size. The Etta mine near Keystone is the most famous for its spodumene: its largest crystal was forty-two feet long, three to six feet across and contained thirty-seven tons of the lithium mineral, and many others nearly as large were mined, the largest crystals of any mineral then known. About a thousand feet west, the Hugo mine also held big spodumene. The Bob Ingersoll mine, two miles west of Keystone, is known for massive ledges of lepidolite and for what is probably the largest beryl crystal ever found, 46 inches in diameter, its full length never uncovered. Commercial output from the district has included mica, spodumene, amblygonite and lepidolite, beryl, columbite-tantalite, wolframite and some cassiterite tin.

It was gold that opened the Hills. Custer's expedition found it in 1874, inside land the Fort Laramie Treaty of 1868 had reserved to the Lakota forever; a rush followed, and after the Great Sioux War the Hills were taken. Pahá Sápa remains sacred to the Lakota.