Binding forces in crystalline solids I am referring to rocks in this instance. Individual crystals grow together in an igneous rock as it solidifies. I know that the $\ce{SiO4}$ tetrahedron is the basic building unit of silicate minerals, and metal cations attach to ensure electroneutrality. 

What I am wondering, however, is what force keeps the crystals together (as in bound to one another) in the solid? For comparison, sediment grains are held in place by a silica or calcite cement in fragmental rocks, but what stops individual crystals from breaking free in crystalline solids?
Converting to answer: While the energy to introduce a grain boundary into a single crystal depends on the relative orientation of the two resulting grains, there will be some atomic rearrangement (strains or reconstruction), and you will end up with a number of bonds across the grain boundary. The boundaries are usually weaker than the bulk crystal, which is why they preferentially fracture along the grain boundary.