Check the listing for details. Pretty Quartz Crystal Cluster Specimen 4.14"/320 Grams. Condition: New. Listed at 13.00 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.