Polarised light micrograph of orthogneiss from the Kesselalm area, Proveis, South Tyrol, Italy. Orthogneiss is a two-mica gneiss of the crystalline basement of the Eastern Alps. It is formed by a moderate metamorphic transformation of igneous rocks such as granite, diorite or syenite. Orthogneiss consists of quartz (colourless), potassium feldspar, plagioclase, mica (brown) and biotite. This stone also contains garnet (splintered grains). The main metamorphosis of orthogneiss was 350-380 million years ago. Magnification: x30 when printed at 12 centimetres wide.