When I was working in art restoration, we took in a broken black marble sink that needed some missing areas replaced. My coworker Ed cast some thin plaques of epoxy tinted with white pigment, smashed them to bits, mixed the bits into a batch of epoxy tinted with black pigment, drizzled on some "veins" of white-tinted epoxy and dragged them through with a toothpick, and then ground and polished the resulting mix. Looked fantastic.