Facts of the Matter
Facts of the Matter
Crystal lattices give
gems unique colorsMinerals in the form of gemstones are beautiful because of what they do to light that passes through them or reflects from their surfaces. There are thousands of different minerals, but only certain varieties of about 15 distinct minerals are prized as gemstones. Of these, some can display many different colors, forms and varieties, a small percentage of which are well-formed, colorful, transparent crystals.
The atoms that make up a crystal are arrayed in a repeating three-dimensional grid. Each mineral has a particular pattern of atoms that we call the "crystal lattice," and each has a unique composition. There can be many different varieties of each mineral, and usually the difference is one of color, such as with emerald and aquamarine, or ruby and sapphire.
Atoms in a crystal lattice are held in place by electrical forces, so the space between atoms is high in electrical potential. Since light is an electromagnetic phenomenon, there is much interaction between the electric field within the crystal and light passing through or reflecting off a crystal. The colors of gems are a result of the interaction of light with the electromagnetic environment inside the crystal.
A transparent crystal's color is a consequence of the atoms in the crystal lattice selectively absorbing some wavelengths of light while transmitting the remainder. We see the transmitted wavelengths as a single color much in the same sense as we hear the blending of musical notes as a single chord.
There are nearly 250 different crystal lattices, and there are many different kinds of atoms that can occupy sites in the lattice. The effect of all possible combinations and variations of 30 or so different kinds of atoms in 200-plus different lattices is that the electromagnetic environment inside each crystal is unique to each mineral variety. And each is unique in the way it interacts with light.
Impurities in colored minerals typically absorb more than one wavelength. The presence of minute numbers of 'foreign' atoms distributed throughout the crystal lattice changes the electromagnetic environment only slightly, but enough to give each mineral species a unique color signature.
The most common coloring impurities are members of a related group of metals known as 'transition' metals, which are responsible in some way for almost all of the colors that we see in naturally occurring rocks and minerals. The primary ones are iron, chromium and manganese, with lesser contributions from cobalt, nickel, copper, vanadium and titanium.
In most minerals, gem quality or otherwise, impurities amount to only five or so atoms out of every million, the equivalent of five people out of the population of the entire state of Hawaii. These "rogue" atoms take the place of other atoms in the crystal lattice as the mineral is forming, thereby distorting the electromagnetic environment within the crystal.
Impurities give color to minerals in several different ways, mostly as a consequence of "loose" electrons associated with atoms of the transition metals. In transition metal atoms the energy levels are relatively close together, and not much energy is required to move electrons to a higher level. Adjacent steps on the energy ladder of electrons in transition metals are in the range that corresponds to the energy of photons of visible light.
When white light passes through a crystal, electrons in the transition metal atoms absorb certain wavelengths, using the energy to power the jump to a higher energy level, or to jump from one atom to an adjacent one. In colorless minerals there are no jumps of energy that correspond to the wavelengths of visible light, so no light is absorbed. In colored minerals there are one or more wavelengths that can be absorbed.
Because there are unique electromagnetic environments inside each crystal, it is not always cut and dry which impurity will impart what color. For example, traces of chromium color rubies red and emeralds green because of the different electromagnetic environments inside the two distinct crystal lattices.
The allure of brightly colored transparent rocks has fascinated and titillated mankind for thousands of years, as evidenced by the gems have adorned people from the earliest times. We are only now understanding the quantum effects that govern electron activity in crystals.
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu