Diatomaceous Earth packaged under our FOSSIL SHELL FLOUR® label comes from an extremely pure fresh water deposit of the diatom Aulacoseira from a deposit created in the Miocene epoch. Why is this diatom so special? Its shell is made of amorphous silica. Its shape and hardness are important to how it works. Its hardness keeps it from dissolving in liquid. The holes along the diatom's wall allow it to absorb moisture, hence its use as an anti-caking agent. Darker colored Diatomaceous Earth often contains an excess of a particular mineral such as iron or a high percentage of clay.