There are powders and gels that some big guys sell that are made for doing this, but in general it is just pulverized stone. A few members here sell it in powdered and granulated form. Basically, the voids are filled with the stone powder, packed down tight, a bit of thin CA to soak in and stabilize, repeat as necessary, then finish turning and sand...There are as many "proper" ways of doing this as there are people doing it, but basically, fill, glue, finish. However you decide to do it, it's just crushed stone.