The Xinyi diamond placer is the only diamond-bearing placer deposit with proven resources in Jiangsu Province. However, the provenance of the diamonds and the exploration clues for potential primary deposits have long remained unresolved. In this study, systematic analyses of garnets from heavy mineral samples in the Chenggang–Xinyi area were conducted to trace the source region and to explore new indicators for primary diamond deposits. Bulk samples from the Xinyi placer (XK01, XK02, and XK03) and artificial heavy mineral concentrates from mafic–ultramafic rocks in the Chenggang Uplift were collected. Major-element compositions of the garnets were quantitatively analyzed using an electron microprobe, yielding 288 valid datasets. The results show that garnets from both the placer and the Chenggang Uplift are similar in composition, dominated by G3- and G4-type garnets, with occurrences of G3D-type garnets. The major-element compositions of garnets from the placer display a significant linear correlation with those from the Chenggang Uplift, indicating consistent elemental trends. This geochemical relationship suggests that the diamonds in the Xinyi placer were derived from metamorphosed pyroxenite within the Chenggang Uplift. Future exploration should therefore focus on identifying eclogite-type (E-type) indicator minerals. The garnets from the placer bulk samples mostly retain angular crystal shapes and resorption features, reflecting short-distance transport and implying that the diamonds were not transported over long distances but originated from local primary sources. These findings provide new mineralogical evidence for constraining the regional diamond metallogenic potential and guiding further exploration in northern Jiangsu.