The black stuff on turf football fields is crumb rubber, which is made of small pieces of styrene-butadiene rubber that are ground up from old vehicle tires. Crumb rubber is used as infill to make the fake fields softer and more durable. It is also used to stabilize the synthetic fibers and ensure they remain upright. The black specks found in some varieties of artificial turf are small particles of rubber, which also gives the turf a more gentle feel underfoot.