Just did a little research...
Hydroton, a.k.a. hydrocorn, are kiln-fired clay pebbles imported from Holland. Their neutral pH and excellent capillary action qualities make them a great hydroponic medium. Preferred by horticulturalists for hydroculture, this product offers many advantages when compared to soil. The medium is clean, odor-free, reusable, and light so as not to crush plants. It offers excellent aeration. Hydrocorn prevents rotting, excess acidity, and soil pests.