As the largest known ancient jade mining site, the Mazongshan Site is crucial for understanding the “West–to–East Jade Transportation” system in ancient China. However, its vast nephrite materials remain poorly characterized mineralogically and geochemically. This study employs a multi-technique approach, including polarized light microscopy, SEM-EDS, XRD, CRS, EPMA, and ICP-MS to analyze Mazongshan nephrite. The results identify tremolite as the principal mineral, with accessory minerals including diopside, apatite, serpentine, calcite, dolomite, graphite, hornblende, epidote, forsterite, and albite, as well as limonite occurring as a secondary mineral formed by oxidation. Its rare earth element patterns show significant negative Eu anomalies, low total REE concentrations, and low levels of Cr, Ni, and Co. These results confirm a metamorphic origin for the deposit. Most significantly, the high compositional affinity it exhibits with Hetian nephrite from Xinjiang, together with evidence of ancient mining, has led us to reconsider the prevalence of nephrite materials used during the Warring States to Han periods.