Hydroton is the brand name for a lightweight clay that is used as growing medium. Made in Germany.
They mine the clay, form it into clay pellets, them fire it in a kiln at really high temperatures to cause it to expand into lightweight clay balls (8-16mm in size).
Resulting in an ideal surface structure that is semi-porous and has a balanced capillary action.
Other cheaper knock-off brands of Hydroton can float and cause issues. Along with irregular formed and broken balls.
To ensure the greatest results, Hydroton should be rinsed before you use it. Can be used multiple times as long as it is cleaned each time.