What is the main constituent of a Pearl ?

The main constituent of a pearl is calcium carbonate (CaCO3). Chemically, pearls are biogenic structures primarily composed of calcium carbonate crystals, specifically in the form of aragonite or calcite polymorphs [t1][t5]. These crystals are held together by an organic matrix of a protein called conchiolin, which acts as a biological cement [t5][t6]. While pearls may contain trace amounts of other elements like magnesium, strontium, or manganese, these are not considered main constituents [t1][t8]. The nacreous layers that give pearls their characteristic luster are almost entirely made of aragonite (calcium carbonate) and conchiolin [t4][t9]. Options involving magnesium carbonate or calcium sulphate are incorrect as these are not the primary structural components of pearls; for instance, gypsum is calcium sulphate and is distinct from the carbonate-based composition of pearls [c1]. Therefore, calcium carbonate is the only primary mineral constituent.

Sources

1. [1] https://www.science.gov/topicpages/c/calcium+carbonate+shells
2. [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC8197316/
3. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 13: Types of Rocks & Rock Cycle > 13.5. Some Rock-Forming Minerals > p. 175