Description Approx. Size: 40-60mm 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. Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones make up about 20 to 25 percent of all sedimentary rocks.
