Minerals displaying little or no lustre are described as dull. The surfaces of these minerals comprise coarse granulations, which scatter light in all directions. Clay minerals, for example, have dull lustre.
A crystal which is of roughly equal size in all directions is described as equant. Although this description implies that equant crystals tend towards a spherical form, the term describes both rounded and angular crystals.
As a mineral grain is rotated under cross-polarised light it will periodically turn black. When the grain is black it is in extinction. The extinction positions occur at every 90° of rotation. This position is measured in degrees from a prominent feature of the mineral grain, such as a cleavage plane or a face of a euhedral crystal.
The fabric of a rock describes the geometric arrangement of its constituent grains. The term is most commonly applied to metamorphic rocks. These may display either linear or planar fabrics, depending on the deviatoric stresses present during their formation.
A silicate mineral containing a high proportion of iron and/or magnesium. These minerals tend to be dark in colour. Common ferromagnesian minerals include olivine, pyroxenes, amphiboles, biotite mica and garnet.