Diatomaceous earth can be calcined or non-calcined. The difference between the two is very important depending on what purpose the DE is being used for.
Calcined diatomaceous earth has been treated at a temperature above 1000 ºC. The purpose of this is to further harden the exoskeletons of the diatoms in order to create a better filtering agent. This process causes the amorphous silica that makes up the exoskeleton of the diatom to turn in to crystalline silica. This is a benefit if the diatomaceous earth is to be used as a filtering aid (for example, in a pool filter), however crystalline silica can be toxic to humans and animals when inhaled. Calcined diatomaceous earth is not used for animal feed and is not food grade.
Natural diatomaceous earth is non-calcined meaning that it has not been treated at a high temperature. The amorphous silica remains in its natural state and is not considered harmful to animal or human health. Diatomaceous earth products, such as Red Lake Diatomaceous Earth, are required to contain less than 1% Crystalline Silica. Calcined DE products however, may contain up to 70% Crystalline Silica!