Diatomite

Diatomite is a chalk-like, soft, friable, earthy, very fine-grained, siliceous sedimentary rock, usually light in color (white if pure, commonly buff to gray in nature, and reportedly rarely even black). It is very finely porous, very low in density (floating on water at least until saturated), and essentially chemically inert in most liquids and gases. It also has low thermal conductivity and a rather high fusion point.
The key to the exceptional properties of diatomite lies more in the microstructure – each diatom is peppered with thousands of holes, usually of three distinct sizes, ranging from a few microns in diameter down to submicron diameters.
The most common use is as a filter medium, especially for swimming pools. It is also used particularly in the drinking water treatment process. Other industries such as paper, paints, ceramics, soap and detergents use it as a fulling material.
Mineralogy Natural silica
Particle Size (µm) 5-100
Typical MOH Hardness 5 – 6
Specific Gravity 2.0 – 2.3
Refractive Index 1.48
Moisture (max) 0.1 – 9
pH 6 – 11