Try burning it if it catches fire it's synthetic rubber natural rubber doesn't burn but does make up 25% of tires. If the debris doesn't burn either natural rub

Try burning it if it catches fire it's synthetic rubber natural rubber doesn't burn but does make up 25% of tires. If the debris doesn't burn either natural rubber or brake dust or a mix. Modern brake pads are ceramic or metal but not asbestos anymore. But any burning of the dust would indicate synthetic rubber is present.