Description 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. Gneiss (/ˈnaɪs/) is a common and widely distributed type of metamorphic rock. Gneiss is formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Orthogneiss is gneiss derived from igneous rock (such as granite). Paragneiss is gneiss derived from sedimentary rock (such as sandstone). Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct foliation. A gneiss in which the dark green or black amphibole mineral hornblende is largely responsible for the dark coloration is typically referred to as a hornblende gneiss Approx: 30-40mm
