A mineral is a natural inorganic substance that has a particular chemical composition, and a regular atomic structure. There are over 2000 different mineral species of which about 360 are found in Ireland. Igneous, sedimentary, and metamorphic rocks are composed of only a very small number of minerals, and these are referred to as being ‘rock-forming’. The silicates, some oxides, and the carbonates are the most important of these.

The following pages outline the characteristics of minerals and crystals and illustrate the more important criteria by which they are distinguished.

Pyrite - interpenetrant twin


Diagnostic Features

Diagnostic features include the crystal form, habit, twinning, aggregate form, basic shape and symmetry.



Orange Fluorite


Physical Properties

Physical properties, which are often characteristic, include hardness, cleavage, fracture, specific gravity, color, streak and lustre.