Mineral hardness is described using the Mohs scale. The scale runs from very soft at 0 up to very hard at 10. The positions of minerals are determined by their relative hardness. For example, quartz can scratch calcite, but calcite cannot scratch quartz. Thus quartz is harder than calcite.

Parallel lamellae or trains of blebs of one mineral, all in the same optical orientation, are enclosed in a single “host” crystal of another mineral. This texture is generally attributed to solid-state exsolution of the daughter phase from the host.

Rocks with a crystalline structure in which the individual crystals cannot be identified by the naked eye, but are large enough to be identified using standard optical microscopy are described as microcrystalline.

A rock containing a significant proportion of micas is described as micaceous. The micas are a group of sheet silicate minerals, which include biotite and muscovite.

Minerals displaying a metal-like lustre are described as metallic. These minerals have surfaces which are perfectly reflective. Pyrite, for example, has metallic lustre.

Mesocratic rocks have a moderate colour index. That is to say that 30-60 vol% of the rock is made up of ferromagnesian minerals.

Melanocratic rocks have a high colour index. That is to say that 60-90 vol% of the rock is made up of ferromagnesian minerals.

A relatively large crystal, usually occurring within an igneous rock. Unlike the term phenocryst, it does not imply a petrogenetic link between the crystal and the surrounding magma.

A combination of the intensity and quality of the light reflected from the surface of a mineral. A number of terms are used to describe the range of lustres displayed by minerals, including adamantine, dull, metallic, pearly, resinous and vitreous.