Nokta FoRs Gold, a Gold Cube, 2 Keene Sluices and Lord only knows how many pans....not to mention a load of other gear my wife still doesn't know about!
If you took the rock on the right and laid it horizontal, there is then a flat top and bottom, and what looks like a part to fit together with another stone. Think along the lines of tongue-in-grove wood.
Kinda like this (apologies for my poor drawing skills):