Description All our rocks are carefully selected for educational study. Average specimen size is shown below. These are ideal for classroom teaching and are a size that can be easily passed round the classroom for students to get a feel for each type. Chert is a hard, fine-grained sedimentary rock composed of microcrystalline crystals of quartz, the mineral form of silicon dioxide. Chert is characteristically of biological origin but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood. Approx: 30-40mm
