How did natural stone tesserae affect the appearance of faces and hands in Byzantine mosaics?

In Byzantine mosaics, natural stone tesserae were specifically chosen for the depiction of faces and hands because their dull, granular surfaces provided a soft, matte appearance that contrasted with the hard brilliance of glass and gold tesserae used elsewhere in the composition.12

By using stone rather than glass for these features, artists were able to achieve hues more reminiscent of actual human skin, which added a sense of "mysterious reality" to the figures. This technique allowed for gentle gradations of color that stood out against the more vibrant, often violent juxtapositions of colored glass used for hair and garments. Furthermore, because these areas required high levels of detail, mosaicists often used smaller-than-average stone tesserae to refine the modeling of facial features and hands. In some instances, such as in the 9th-century works at Istanbul, artists even painted marble cubes to achieve specific skin tones, further enhancing the lifelike quality of the subjects.342