Figure 61 (a) A classic rhombohedral cleavage fragment of calcite (crystal 5 cm across). (b) Plane-polarised light view of calcite crystals in a marble (field of view 2 mm across). Because calcite has high anisotropy, both cleavage and twin planes can be observed in the crystals. (c) The same field of view as in (b) between crossed polars. Calcite has extremely high interference colours and these fourth-order colours have distinctive 'washed-out' pastel tones.