Twinning is often seen and results in crystals with blocky chevrons, right angled prisms, heart shapes or dipyramidal shapes. A notch in the middle of a doubly terminated scalenohedron is a sure sign of a twinned crystal. Pseudomorphs after many minerals are known, but easily identified as calcite. Also massive, fibrous, concretionary, stalactitic, nodular, oolitic, stellate, dendritic, granular, layered, etc. etc. Cleavage is perfect in three directions, forming rhombohedrons.
Fracture is conchoidal.
Streak is white. Best Field Indicators are crystal habit, reaction to acid, abundance, hardness, double refraction and especially cleavage. 1998,1999 by Amethyst Galleries, Inc.