Polarised light micrograph of moss agate, or mocha stone, a semi precious gemstone formed from silicon dioxide. It is neither a true form of agate, nor does it contain moss. It is a form of chalcedony and does not exhibit the concentric banding shown in true forms of agate but contains inclusions, formed by impurities such as iron and chromium, which resemble moss and give it its name. Magnification: x27 at 10cm wide.