Seen at the Denver Museum of Nature and Science, it’s the Alma King — a fantastic rhodochrosite specimen:
Strictly speaking, rhodochrosite is a manganese ore, used in the production of stainless steel and some aluminum alloys. But in a few places, gem-quality rhodochrosite crystals can be found. The Sweet Home Mine near Alma, Colorado (about 144 km / 90 miles southwest of Denver) is one of the best sources of these cherry-red gems.
Discovered in the Sweet Home Mine in 1992, the Alma King is the largest rhodochrosite crystal ever found — 14 x 16.5 cm (5.5 x 6.5 inches) in size, and widely considered the most valuable mineral specimen ever mined in North America. Largely due to the Alma King’s discovery, rhodochrosite was named the Colorado State Mineral in 2002.