A slab of Rhodochrosite on display at the natural history museum in D.C.:
This mineral is not found in too many places on Earth… one really good (now played out) mine in Colorado, China, and where this one came from, Argentina. The Colorado crystals were typically rhomboid cubes, making great gems, but the Argentine stuff is more like what you see here. Rather than clear and flawless, it’s layered. But it also can be pretty enormous… this is a slice of stalactites (or stalagmites, unclear), which means there was a cave jam-packed with this stuff until someone came through and sawed it all apart.
By far the best collection of the stuff is in the Denver museum of natural history (or whatever it’s called these days). There is a sizable *wall* covered with gem-quality rhodochrosite crystals, one one truly enormous cube of the stuff.