Thin sections of silica deposits in fractures. (a) Horizontal fracture developed by breakage along syntaxial overgrowth contacts and filled with opal cementing tiny quartz splinters (polarised light). (b) Thin section cut parallel to a glossy silica coating a vertical fracture; the quartz grains are corroded and cemented by opal (polarised light). (c) As for (b) with quartz wedge plate. (1) Quartz grain; (2) Opal cement.