Growstones are a substrate for growing plants that can be used for soilless purposes or as a soil conditioner.
Another property of this medium is that it holds more water than parboiled rice hulls.
[1][2] Growstones appear to be a comparable alternative to expanded clay aggregate.
Growstones are made in a variety of sizes from a large 1 to 2 inch sized "Lift" product made to supplement drainage in potted plants while retaining superior moisture and nutrients for the plant roots, to "Gnat Nix" which is a small "5MM" size product to prevent fungus gnats from thriving in potted plants.
Growstone ceased operations as of January 26, 2019, though warehoused product is still available.