Abstract: Diamonds have been picked out from the gray-green and purple-red sedimentary interlayer of the Cretaceous sedimentary red basin in Ligonggang area of Taoyuan County, Hunan Province. Taking the gray-green rock as the research object, the authors determined the rock name and the property of garnet and mica in rock through thin section identification and electron probe analysis. Under the microscope, the gray-green rock has a tuffaceous structure, and it is composed of 45% crystal fragments, 35% rock cuttings and 20% vitric fragments, which is the typical sedimentary tuff rather than the kimberlite identified by predecessors. The garnet and mica in sedimentary tuff are respectively G10 pyrope and phlogopite. The chemical composition characteristics of G10 pyrope and phlogopite are similar to the diamond-bearing kimberlite of typical rock tubes from Liaoning, Shandong and South Africa, which indicates that pyrope and phlogopite are from the diamond-bearing kimberlite. The characteristics of the sedimentary tuff indicate that the tuffaceous substance source is not too far, which lays a foundation for finding volcanic edifice in the neighborhood. The discovery of kimberly rock cuttings, G10 pyrope and phlogopite provides valuable information for searching the primary kimberlite in the surrounding areas.