In addition to crystal form, colour is one of the other main reasons why collectors say they collect minerals. Yet how many know that there are four different processes which give rise to mineral colour. We discuss these briefly in this article.

Colours from Charge Transfer

Sometimes a mineral will contain two different ion types of the same metal in the crystal structure. In most cases this combination tends to be Fe2+ and Fe3+ ocurring in adjacent lattice sites. Light causes an electron to jump from one to the other causing a switch in electric charge between them. As the light absorbed is in the red part of the spectrum the resulting mineral colour is usually blue, as in Rockbridgeite.

Interestingly the absorbtion tends to depend on the direction of a crystal in relation to the light source, producing strong pleochroism (different colours according to direction of crystal) in these minerals. Ocassionally the colour is caused by charge transfer between different metal ions. Thus we have Fe2+ and Ti4+ in Pyroxenes and Mn2+ and Ti4+ in Elbaite.

Colours from Metal Ions

This is the most common source of colour in minerals. Charged metal ions have discrete energy bands between which electrons can jump if given sufficient energy. This is usually in the form of light, meaning that the colour corresponds to the energy gap between the levels. Small gaps require little energy so less energetic red light will produce a transition. Large gaps need high energy blue or violet-coloured light.

The resulting mineral colour is thus produced as the visible spectrum minus the colour of the absorbed light. Significantly the size of the gap is greatly influenced by the surroundings environment of the ion. Thus an Fe2+ ion in a square complex produces a bright red colour, whereas the same Fe2+ ion in an octahedral complex produces a deep green.

Other notable metal ion colours include: V3+ in garnets produces greens. Cr3+ causes the green of emeralds but reds in ruby and spinel. Mn3+ causes reds and greens but Mn2+ usually gives violets. Co2+ in tertrahedral sites causes blues. Ni2+ is green in octahedral sites but yellow in other structures.

Colours from Radiation

Ionising radiation which has large amounts of energy can radically alter the internal structure of a crystal. As it passes through it can knock entire atoms out of place, creating what are known as f-centres in which loose electrons can get trapped. These absorb various wavelengths of light giving rise to a wide range of colour. Examples are coloured Diamonds, the wide range of coloured Fluorites, blue Calcite, and many of the pink Tourmalines.

Colours from Physical Effects

Many colours are caused by the scattering and interference as light passes through and interacts with a mineral. Examples include irridescence, caused by a difference in refractive index of crystal layers, and diffraction caused by regularly spaced layers which separete wavelengths of light. Finally, come inclusions which are foreign materials in the crystal alttice. For example, the black colour of some Plagioclases is due to inclusions of Magnetite.