one way can be this:

Download 3D rocks models and convert these quad mesh topology objects with Blender (a free program) and then import them into Moi3D as subdivision surfaces.
tutorial triangulated mesh to quad



and with another free application


Be careful not to use a high resolution.

Here in this link you can download rocks suitable to be imported into Moi3D for free.
https://free3d.com/3d-model/rock-boulder-and-stone-collection-236212.html

Below those rocks imported as SubD without remeshing because they are ready with quad topology.



Greetings.