Diatomaceous Earth packaged under Perma-Guard's 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. From Perma-Guard: With more than 600 deposits of diatomite west of the Mississippi—and only four  to our knowledge  that can be considered food grade it is very important that people know what they are using. The vast majority of diatomite deposits are impure. When diatomite is formed  water currents bring in foreign material that mixes with the microscopic diatom shells. This type of diatomite is absolutely without value for practical purposes  but many people tend to think that if it is “Diatomaceous Earth ” then it has the same value as other  purer deposits. There are many places where diatomite deposits can be loaded up “for free ” but generally this substance has no value. If this impure diatomite is sold for practical usage  the customer may end up with an unpleasant surprise.