How to tell them apart
Calcite cleaves perfectly into rhombohedra. Aragonite has poor cleavage and fractures conchoidally. Calcite's hardness is 3; aragonite is 3.5–4. Calcite fizzes vigorously in dilute HCl; aragonite reacts more slowly. Calcite's specific gravity is 2.71; aragonite is 2.95 (you can feel the heft difference once you've handled both).
Why two polymorphs exist
Aragonite is metastable at surface temperature and pressure but kinetically locked into its structure — meaning it doesn't spontaneously convert to calcite even though it 'should' thermodynamically. Geological time and warm groundwater are enough to drive the conversion; many ancient aragonite shells are now calcite. Modern shells, sputnik clusters, and hot-spring deposits remain aragonite.
Famous aragonite localities
Molina de Aragón, Spain (the type locality, source of the name) — produces classic cyclic-twinned 'sputnik' aragonite. Morocco (Sefrou region) — fan-like aragonite sprays in stunning natural color. Tsumeb, Namibia — small but exquisite aragonite groups. Sicily — flos-ferri aragonite (coralloid aggregates). Each has a distinctive habit.