Salivary calcium is a biologically plausible but methodologically challenging biomarker because of its central role in enamel demineralization/remineralization, dental calculus formation, and oral mineral homeostasis. This narrative review summarizes the analytical challenges and clinical interpretation of salivary calcium measurements in oral and systemic disease. Evidence from periodontal research generally indicates higher salivary calcium concentrations in periodontitis, although interpretation is complicated by calculus formation, gingival crevicular fluid contribution, smoking, inflammation, and variation in saliva collection protocols. In dental caries, findings are heterogeneous, and calcium appears more meaningful when interpreted together with salivary flow rate, pH, phosphate, buffering capacity, and fluoride exposure than as an isolated diagnostic marker. Studies in osteoporosis, chronic kidney disease, hemodialysis, and diabetes suggest possible associations with systemic disturbances, but salivary calcium does not currently provide sufficient specificity or predictive value to replace established serum, imaging, or clinical biomarkers. Overall, salivary calcium should be regarded as a context-dependent component of broader salivary biomarker panels. Its clinical translation requires standardized sampling, clear distinction between total and ionized calcium, appropriate analytical validation, and careful control of pre-analytical and biological confounders.