What are the Diatom and Diatomite? Diatomite(DE) is a chalk-like,soft,friable,earthy,very fine-grained,siliceous sedimentary fossil rock,usually light in color(whiht if pure,commonly buff to gray in awter,and rarely black).It is very finely porous,very low in density(floating on water).It also has extremely low thermal conductivity and a rather high fusion point 1200℃.Calcined DE is used in filter and absorbent field.