The genus Lithops is an amazing group of succulents native to harsh desert regions in southern Africa, and this set of 37 species rightfully earns its Latin name, which means "stone face”. Their highly modified leaves are fused, pigmented, and textured such that the top face of the plant—called the window—resembles the mottled patterns of small rocks or fields of pebbles, though admittedly some look more like brains. Multiple patterns have evolved repeatedly across species, and even more extraordinarily, the window is actually just that. Because the pigmented exterior surface of the leaves is not photosynthetic, the window surface cells and those below them have evolved to be translucent. Thus, some light can pass down into the leaf interior, and the cells that line the lower interior surface of the leaf conduct photosynthesis underground.

--Ben Blackman

Image from here and  Lithops Dissection