Quartz sand temper in a Dogoszhi Black-on-white sherd at 40x magnification. the Gray color at the bottom of the photo is "carbon streak," a charateristic feature of this pottery ware.
Below: Quartz sand in Dogoszhi black-on-white sherd at 10x. Note the carbon streak in center of the sherd's wall.
Compare with asbestos floor tile at 10x (below). Note the absence of carbon streak, irregular sizes and shapes of inclusions.