I'd like to build an assembly (for lack of a better word) made out of river rocks similar to that I saw in a new park near my home. The are put together in an approximate conical shape as in the attached photo.
The rocks are bonded together with a bonding material placed very sparingly in the back of the stones and somewhat hidden from sight so as to give the composition a dry stack look.

My calls to the landscaping contractor that did the work with a request to share with me the bonding material they used were not returned. A close look between the rocks shows that the surface of the bonding material is very smooth which, in my view, excludes cementitious mortar. It is grey in color and very hard on contact (I could not scratch it with my fingernalis as much as I tried).

Given the large size of some of the stones, their irregular shapes and disposition the bonding material woud have a fast setting time. Even so I'm still wondering how they could stay together without, initially, being held in place perhaps manually.

Any suggestion as to the bonding material and work execution is very much appreciated.

Thanks