Check the listing for details. Calcite Crystal Cluster Specimen 6.65"/1539 Grams. Condition: New. Listed at 42.00 USD. A calcite crystal cluster is a formation of multiple calcite crystals grouped together, typically exhibiting a range of shapes and sizes. Calcite, a carbonate mineral composed of calcium carbonate, can crystallize in several habits, including rhombohedral, scalenohedral, and prismatic forms.