The use of tire chips (also known as rubber rock) in septic systems, as an alternative to stone, has become a common practice. Tire chips are cheap, lightweight (meaning they are easier than stone to transport) and are a great way to recycle old tires. Locally, tire chips are available from Entech in White Pigeon Michigan.
The Indiana State Department of Health allows the use of tire chips. See the link below to read their policy.