You are correct: Steatite was not the stone I was thinking of. Rather: “Most statuary was painted; even stones selected for the symbolism of their color were often painted. For instance, the exemplary statues of Menkaure, builder of the smallest of the three major pyramids at Giza, were executed in dark schist (also called graywacke). This smooth black stone is connected with Osiris, resurrected god of the dead who was often shown with black or green skin referring to the fertile silt and lush vegetation of the Nile valley.
These images preserve traces of red paint on the king’s skin indicating that, when completed and placed in his memorial temple near his pyramid, they would have appeared lifelike in coloration. With time, the paint would have flaked away, revealing the black stone underneath and explicitly linking the deceased king with the Lord of the Underworld.