Native to the Cape Province of South Africa, Pleiospilos nelii or split rock has a number of other relatives that resemble rocks or pebbles. These include living stones (Lithops spp. and relatives).
For a few other images of why this plant deserves its common name, an image search on Google for Pleiospilos nelii has a number of results.
Botany resource link: Learn more about the tropical genus of plants Peperomia at peperomia.net–“The Internet Peperomia Reference”.