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Gypsum is primarily made of calcium sulfate hemihydrate, which is a key component in various dental materials, including dental stone and plaster. This specific mineral occurs naturally as the mineral gypsum, and when ground and heated, it loses water molecules to form this hemihydrate phase. The importance of calcium sulfate hemihydrate lies in its ability to set and harden when mixed with water, making it essential for creating accurate impressions and models in dental applications.

Other options do not reflect the composition of gypsum: for instance, calcium carbonate is commonly found in limestone and is not related to the properties of gypsum used in dentistry. Calcium phosphate is a prevalent mineral in bone and teeth but does not pertain to gypsum. Silica gel is a desiccant used for moisture control and does not form part of gypsum's chemical composition. Understanding these distinctions clarifies why calcium sulfate hemihydrate is the correct answer in the context of dental hygiene and material science.