Color

The color of an object (mineral) is not a true material property. Color is a perception and depends on the light source and our visual system. When a mineral (e.g., azurite) is irradiated by a light source (photon source), such as the sun, certain atoms/ions, or rather their electrons, are excited within the crystal structure and raised to a higher energy level. In azurite, this occurs primarily in the d-orbitals of the ^Cu²⁺ ions. If the energy of a photon corresponds exactly to the difference between two energy levels of an electron, the photon is absorbed, and the electron jumps to the higher energy level. Photons with other energies (wavelengths) are not absorbed and are instead reflected. In azurite, these ^Cu²⁺ ions absorb photons from the red and yellow regions of the light spectrum. The remaining light (photons), which consists mainly of blue wavelengths, is emitted and appears blue to our eyes.

Another cause of mineral color is lattice defects in their crystal structure, known as color centers. A common example is F-centers, which arise when certain atoms/ions are missing (vacancies) in the mineral's structure. These vacancies are filled by one or more electrons, which cause specific absorption of certain wavelengths, thus coloring the mineral. A frequent cause of these lattice defects in nature is radioactive irradiation (mostly alpha particles), for example, by associated minerals, with radioactive nuclides within their structure. Other examples of color centers are M-, R-, or N-centers.

Minerals are generally classified as either idiochromatic or allochromatic. Idiochromatic minerals have a distinct and characteristic color due to their chemical composition. This means that the color results from the presence of specific chemical elements that are integral to the mineral's crystal structure. Malachite is an example of an idiochromatic mineral; its characteristic green color is due to the presence of copper, which is also a fixed component of the crystal structure. In contrast, allochromatic minerals have variable colors due to trace elements, inclusions, or defects in the crystal structure. Quartz, for example, can exhibit different colors depending on the type of impurities and its purity.

In some cases, sunlight can affect the color changes of minerals, especially the high-energy UV component. This radiation with such short wavelengths can destroy color centers, reduce or oxidize ions (e.g., ^Fe²⁺ to ^Fe³⁺ ), or even break strong chemical bonds in the crystal structure. To avoid discoloration from sunlight, minerals should ideally not be exposed to direct sunlight and should be stored in dark or cool rooms.