Ah, sorry GF, I believe they contain a level of silica, but they are supeheated shale. Here is the product. They hold water like hydroton, but don't roll and supposedly over time release trace amounts of a couple minerals.
The cost was a bit cheaper than the hydro, so I went for it.
Sunleaves :: Growing Media
There is not enough room in the tray for an air stone as the water will stand at about 1/8 inch. I do like the idea of using tile squares to keep the roots from laying in the water.
My assumption is that once the roots get big enough, they will draw the remaining water from the tray between flood cycles.
I am anxious to see how the flood cycles need adjustment as the plants grow.