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Background: Calcium-rich animal and mineral materials, grouped in Rasa Shastra as Sudha varga, are incinerated to prepare bhasmas such as Shankha (conch), Shukti (oyster or pearl oyster shell), Pravala (coral), Mukta (pearl), Varatika (cowry) and Kukkutanda twak (eggshell). These are increasingly marketed as natural calcium supplements. Objective: To review the chemical composition of calcium-based bhasmas, the factors that govern their calcium bioavailability, and the evidence for their clinical use, and to compare them with conventional calcium supplements. Methods: A critical narrative review of physicochemical characterisation studies, together with established literature on calcium absorption, requirements and supplement safety. Results: Calcination converts biogenic aragonite to calcite and removes the organic matrix; with stronger or repeated heating, part of the carbonate is converted to calcium oxide and then calcium hydroxide (portlandite). In Mukta bhasma, calcium content rose from 34.1% in raw pearl to 46.3%, above the 40% expected for pure calcium carbonate, consistent with partial decarbonation. Eggshell bhasma has been reported as mainly calcium hydroxide. At classical doses of a few hundred milligrams per day, these bhasmas supply only a small fraction of the adult calcium requirement. Their absorption is expected to follow that of the corresponding calcium salt, and no comparative human absorption study was identified. Conclusion: Calcium bhasmas are best understood as calcium carbonate, oxide or hydroxide of biogenic origin. Claims of superior bioavailability are unproven, and their phase composition, calcium content and heavy metal levels should be stated on labels.