The habit catalog
Rhombohedral (cleavage rhombs, nailhead form). Scalenohedral (dogtooth — six tapering triangular faces meeting at sharp points). Prismatic (long hexagonal columns). Tabular (thin plates). Acicular (needles). Botryoidal. Stalactitic (flowstone, dripstone). Coralloid. And dozens of combination forms where two or more dominant faces meet on one crystal.
Distinguishing calcite from aragonite and dolomite
Calcite (CaCO₃ trigonal) cleaves rhombohedrally. Aragonite (same formula but orthorhombic) does not — it fractures conchoidally. Dolomite (CaMg(CO₃)₂) is rhombohedral like calcite but harder (4 vs. 3) and fizzes much more slowly in dilute HCl because the magnesium slows the reaction. All three are common at Chinese localities; knowing the difference matters for accurate IDs.
Locality flavor
Daye-Hubei produces calcite on golden pyrite plates, often with iridescent surfaces. Wenshan, Yunnan, produces deep amber 'Wenshan' calcite scalenohedrons. Romanian Cavnic produces stalactitic flowstone. Mexican Naica produces gigantic gypsum-and-calcite chambers. American Tri-State produces the famous twinned 'butterfly' calcites. Each locality is a distinctive look.