Photomicrograph of fibrous silica (SiO2). The fibrous texture is a consequence of intense shear-forces exerted on this siliceous rock, forces that also account for the undulation of the fibrous mass. A widely known variety of a rock formed by fibrous silica is tiger-eye, a semi precious stone used in jewelry and all manner of trinkets. Thin section, transmitted polarized light. Actual size 27 x 35 mm