Stone Forest calls their bathtub "Natural" for its raw design, a rugged just-rolled-down-the-mountain aesthetic, rather than the precision-cut and polished-to-cold-industrial-modernity one we might expect from this type of soaking, personal hygiene, and (at least occasionally, I hope) dirty deeds basin. Not that the Santa Fe-based company isn't capable of both. Founded by former climbing and river guide Michael Zimber, Stone Forest has rolled out a whole line of rock-based tubs and sinks. They carve their Natural Bathtubs from a single block of granite or marble.
Granite tubs look like the one in the photo above--chiseled and nubby on the outside with smooth interiors. Marble tubs get a polish all over. Stone Forest has stock slabs of both with specifications for 60" and 72" tubs, but all of them are carved to order with your preferred customizations in style, size, and drain configuration.
One word of advice: you might want to add some structural reinforcement to your floors before installing a Stone Forest build--they weigh anywhere from 1600 to 2600 pounds.