Cryptocrystalline silica. Composed of extremely fine (submicroscopic) silica crystals
US Department of Labor, Silica eTool - Glossary, Occupational Safety & Health Administration OSHA, Washington, D.C., USA 2010. https://www.osha.gov/dsg/etools/silica/glossary/gl ossary.html, (OSHA/ Silica - Amer) {625}