The Causes of Color in Minerals and Gemstones, Part 1,
By Paul F. Hlava, Mineralogist

Quiz?

What colors would you expect minerals to be that contain:
Iron - Fe2+
Iron - Fe3+
Cobalt - Co
Sulphur - S
Copper - Cu
What causes the color in yellow sapphire?

Purposes/Goals/Outline

 Discuss the nature of light a little bit.
 Mention about our perception of color.
 Introduce the 15 causes of color.
 Describe Transition Metal Absorption Colors and  use ruby, emerald, and alexandrite as examples.
 Describe Intervalence Charge-Transfer Color and  use sapphire, amazonite, and lazurite as examples.
 Light is Part of the Electromagnetic Spectrum

 Light is a from of energy that makes up part of the electromagnetic spectrum. Visible light, however, comprises only a small part of what is referred to as the electromagnetic spectrum. Other parts of the spectrum include Infrared (heat), Ultraviolet, X-Rays, Gamma rays, and Cosmic Rays. Generally, the higher the frequency, the higher the energy. Light is a wave, just like a radio wave, but it also can behave simultaneously like a particle known as a photon.

Seven colors comprise the spectrum: red, orange, yellow, green, blue, indigo, and violet. The wavelengths of white light may be divided into:
Red 700.0 nm to 640.0 nm
Orange 640.0 nm to 595.0 nm
Yellow 595.0 nm to 575.0 nm
Green 575.0 nm to 500.0 nm
Blue 500.0 nm to 440.0 nm
Indigo
Violet 440.0 nm to 400.0 nm

Light is also a Form of Energy

In addition to wavelength and frequency, colors have energy units associated with them. Infrared and red light represents low energy, and ultraviolet and violet light denotes high energy. Color can be used to determine the temperature (a measure of energy) in black bodies or black iron bars that are heated.

Perception of Color

We each see light in our own way. If a number of sounds are mixed, one can usually distinguish the various frequencies coming from different sources (i.e. different instruments in an orchestra). However, mixed frequencies of color produce one perceived final color. The same final color can be achieved by several different combinations of color. The wavelength of violet light, for example, is about half that of red light.

Same Color By Different Paths

An illustration showing three ways of achieving the same color - pink. In the diagram A, light that is orange is mixed with white light to make pink. In diagram B, red light is mixed with a blue-green light to make pink. In diagram C, violet, green, and red are mixed to make pink.

Human Eyes

Human eyes contain rods, which “see” black and white, and three sets of cones (red, green, and blue) that produce all the colors by mixing their signals. The eyes of animals contain more rods than do human eyes to detect movement (hunter versus prey). When you mix frequencies of light, the human eye perceives only one color. In color-blindness, the eye loses its ability to distinguish red, because red light has the weakest and lowest energy. Blue light has the strongest energy.

The Human Eye is Most Sensitive in the Center of the Solar Spectrum.

CIE* Chromaticity Diagram
* Commission Internationale de l’Eclairage

Since the color of light perceived may reflect a mixture of different light frequencies, we need a way of measuring color that is independent of frequency. The Chromaticity Diagram was established in 1931 as an international standard for color measurement. Just so you know, a science of color measurement exists. A chromaticity diagram chart can be set up to define any color on the basis of CIE coordinates, which are often used in technical literature and in correspondence between scientists.

The 15 Causes of Color

Vibrations and Simple Excitations

Here, the electrons simply get excited and, upon falling back to their normal state, give off light. The color is proportional to the distance that the electron fell.

 Incandescence - Flames, Lamps, Carbon Arc.

 Gas Excitations - Vapor Lamps, Lightning, Auroras, some LASERS.

 Vibrations and Rotations - Water, Ice, Iodine, Blue Gas Flame.

 Translations Involving Ligand Field Effects

A ligand is a bonding field between atoms. Most elements found in minerals do not produce color because their electronic configurations remain stable. Silicon, aluminum, calcium, lithium, beryllium, boron, carbon, nitrogen, etc. remain white and colorless.

Transition (and R.E.E. or rare earth elements) metals have inner shells that are not filled. When the shells are filled, the electrons pair up to have a positive and a negative direction. The transition metals occupy different places and are un-paired. They become excitable, are then mobile, and soon available.

Valence electrons from these shells are un-paired and can be excited into unstable levels, absorbing energy to do so, and, thereby, creating colors. Sometimes they can absorb invisible colors, like infrared and ultraviolet, but they usually absorb colors in the visible spectrum. Chrome imparts red in rubies and green in emeralds. The same ion or coordination number on the C.I.E diagram that shows the oxygen surrounding ruby and emerald are in the form of an octahedron. However, the transition levels are different. For example, emeralds fluoresce red. Also, vanadium introduced into corundum makes synthetic alexandrite.

 Transition Metal Compounds - Turquoise, Azurite Malachite, and many other pigments.

 Transition Metal Impurities - Ruby, Emerald, Alexandrite, and some Tourmalines, Red Iron Ore, and many others.

Transitions Between Molecular Orbitals

 Organic Compounds - Most Dyes, Biological Colorants, and some Fluorescence and Lasers.

 Charge Transfer - Blue Sapphire, Lapis Lazuli, Amethyst, Magnetite, and many other pigments.

 Charge transfer or intervalence charge transfer appears when an electron arrives at the valence stand of one atom and moves to the valence stand of another atom. Atoms of titanium and iron bounce ions back and forth to make sapphire appear blue. In the same way, sulphur atoms also bounce ions around to impart the intense blue color seen in lapis lazuli. Amethyst is yet another example of charge transfer, where the iron atoms perform the same activity and affect the color.

The color in both amethyst and citrine are due to impurities of iron in part per million kind of levels. The iron substitutes for silica in the quartz structure. The iron is three valent and gives the yellow color seen in citrine. If it is irradiated, most of it will turn purple, but not all will change color. Perhaps, the presence of hydroxals keeps the iron in the three valent state. Ametrine is a trigonal mineral, a three-fold mineral (not hexagonal, but it appears as hexagonal) with two pairs of zones that alternate amethyst and citrine color.

Transitions Involving Energy Bands

Examples of transitions involving energy bands are:
Metals - Copper, Silver, Gold, Brass, and “Ruby” Glass.
 Pure Semiconductors - Silicon, Diamond, Galena, Cinnabar.
 Doped Semiconductors - Blue and Yellow Diamond, LED’s, and some Lasers and Phosphors.
 Color Centers - Smoky Quartz and Desert “Amethyst” Glass.
Geometrical and Physical Optics

 Dispersive Refraction, Polarization, etc. - Rainbows, Halos, Sun Dogs, and Fire in Gems.
 Scattering - Star and Cat’s Eye Gems, Rainbow Moonstone and Obsidian, Blue Sky, Red Sunsets, Blue Moon, Blue Eyes and other biological colors, and Raman Scattering.

 Interference - Labradorite, Oil Slick on Water, Soap Bubbles, Camera Lens Coatings, and some Biological Colors

 Diffraction - Opal, Diffraction Gratings, Aureole, Glory (the spray of rays from clouds on the horizon at sunrise and at sunset), some biological colors, and most liquid crystals.
In Paul’s presentation, he also talked about the causes of color in emerald, ruby, and alexandrite. He used some black lights to show how each interacted and fluoresced. Paul, a fountain of technical information, went into more detail than we can report here. You really had to be there.

Did you know....

When sapphire is heat-treated, the impurities, like ilminite, are melted, and the electrons intermingle within the corundum. In padparadcha, two coloring agents are at work at the same time. The pink and orange color is due to the presence of manganese and chrome.

Tourmaline can have up to four coloring agents working simultaneously to produce color. Tourmaline, being a non-central symmetric structure, has a crystal stop in bi-colored material. Along the C axis is a line marking the boundaries where manganese produces pink and iron produces green. This is an example of crystal field effects.

Crystal field effects are also found in peridot. In the volcanic melt, olivine with high concentrations of nickel appear in the first stages of crystallization. Distorted sites in peridot are responsible for nickel to quickly diminish in concentration during the melt.

Most Colors are Produced by Absorption

Most colors are caused by electrons interacting, mostly absorbing, some wavelength of light. Only the color not directly caused by electrons is the color of water and ice, which are colored by molecular vibrations. White light is a mixture of all the visible colors. Most colors are produced by the absorption of light. If something removes specific energies from the white light, then the complimentary color remains. Electrons in atomic orbitals “just happen” to do exactly that when they become excited. Electrons are responsible for fourteen causes of color. Where the human eye detects color in the narrow band of the light spectrum, which is mostly invisible, is where the transitions of electrons occur.

More Quiz?
 What makes the sky blue?
 What is opalescence?
 What causes the moonstone effect?
 What causes color in diamonds?
 What causes play of colors?
 What cause “fire” in diamonds?