Ranch below and the ... darn, what is that white stuff in potting soil? Oh, vermiculite? ... nope PERLITE is the white stuff you see. Mineralization is perlite hosted in Arnett Rhyolite. Glassy perlitic flows underlie glassy rhyolite and overlie tuff and breccia. Perlite is light gray to milky white. Strongly developed spheroidal perlitic structures, from a millimeter to several tens of centimeters in diameter, are successfully superimposed on one another. The main mass of perlite contains several percent water.