A rock, high in amorphous silica content, formed from the shells of tiny fresh- and salt-water organisms called radiolaria
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}