Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Desert Rocks Free.
Desert Rocks v1.0
This Pack contains 11 unique meshes with 9 clusters prefabed and ready to go. As well as 2 complemetary tiled textures for your terrain.
All 11 meshes are on a single 2k texture map. Included in this pack are:
-detail Normal (512)
The rocks range from 200-850 tris each and the sum of all meshes is about 4,600 tris.