Diatomaceous earth looks like crushed limestone but isn't. You may find it useful nonetheless.
Diatomaceous earth is composed of the silica skeletons of diatoms.
The clay-like form is finely powdered and is used both as a polishing agent and as a filter medium. The rock-like form is crushed and sold both as kitty litter and as an oil absorbent for garage use. One common name is oil-dri. Diatomaceous earth has lots of surface area, like zeolite clays. When mined, it has rather a lot of trace mineral ions already adsorbed so it is often used as a trace mineral amendment.