The same composition as perlite but in larger chunks - end even larger than the #4 grade. Sponge Rock is a naturally occurring, mixed glassy silicate of volcanic origin which 'pops' when furnaced. Its unique internal structure consists of numerous tiny closed air cells surrounded by a large exterior irregular surface. Sponge Rock is ultra-lightweight, inert, sterile, permanent, incombustible, asbestos- free, non-toxic, rot and vermin proof and has a neutral pH. Its extremely large, open surface area allows it to retain water in mixes while maintaining air spaces, important for healthy root growth.
