Rhodochrosite has a trigonal crystal system. It is a carbonate mineral and is formed in the oxidation zone of manganese ore deposits as a secondary process. Water that contained manganese and carbonic acid collected in hollows of the ore deposits and formed stalagmites and also filled in cracks in the ore.
Deep pink crystals up to several inches across have been found in Colorado, but generally in the US, most of what is found is a milky pink color that can also have pyrite, fluorite or quartz inclusions. Many deep red, almost transparent, nicely banded crystals have come from Argentina. There are also some with a very thin quartz coating that come from Cananea, Mexico.