What I do is mix the gravel (usually stuff I pick up and sift from the parking lot) with molding plaster and pour it into a mold of whatever it is I am making i.e. building foundation, culvert, etc.. When set, I remove it from the mold and then scrub the face with a stiff brush until the desired effect is achieved. Then I stain the "mortar" lines whatever color I need. Molding plaster is used because it is softer, the drawback being that large castings must be reinforced. The staining is done at the end rather than into the plaster during casting because various dyes and staining agents can alter the casting time and its strength. An old but effective trick.