A.L. Bush, in Encyclopedia of Materials: Science and Technology, 2001 (a) Perlite Perlite, obsidian, and pitchstone are volcanic glasses that differ in water content: obsidian, less than 2% H 2 O; perlite, 2–5%; and pitchstone, more than 5%. The general range of composition (Bush 1973, p. 341) is 70–75% SiO 2, and about 10–15% Al 2 O 3.The water content is critical to …What is Perlite? What is its use in the garden? | Trees · Perlite vs. Diatomaceous Earth. Diatomaceous earth is also a mineral additive, available in a fine powder form. It is commonly referred to as DE. DE is used more for pest control than anything else in gardening. It also has high water retention abilities. But since it is a powder, it doesn''t help much with aeration.