The colors we observe from surrounding objects are fascinating phenomena of their interaction with light. When a beam of light strikes an object, a portion of the light frequencies is absorbed by the object, whereas the unabsorbed wavelengths are reflected back to our eyes. The light receptors inside our eyes convey these messages to the brain, producing our perception of a color associated with the object.1,2 Light consists of a continuum of frequencies with corresponding wavelengths and energy levels. The relationship between energy and wavelength can be explained by the equation: E=hc⁄λ, in which λ is the wavelength value, E is the energy, c is the speed of light and h is Plank’s constant.3,4,5 The frequency value is reciprocally proportional to wavelength through the equation v=c⁄λ. (Figure 1a). As frequency increases for an incidence of light, shorter wavelengths are present with greater energy (Figure 1b).
Human eyes are only able to process light and color within a certain range of wavelengths (400-700 nm), the visible spectrum region. The colors detectable by our eyes in this region are red, orange, yellow, green, blue, and violet (from the longest to shortest wavelength).6 Colorless and transparent objects absorb wavelengths outside of the visible region, thus their light reflection is not recognizable as any color by our eyes. Black objects are perceived as possessing an absence of colors by our brain because they absorb all of the wavelengths in this region and reflect nothing back.
An object’s color absorbance and reflection properties are dictated by its molecular structure and composition.7 Atoms consist of a positively charged central core and a cloud of electrons which are negatively charged particles.8 Around the central core, electrons move in a region of space called orbitals, which possess different shapes and energy levels and are responsible for bonding between elements.8,9
In transition metals, there are five d-orbitals that are normally equal in energy.9 When a metal is suspended in a crystal field (a gemstone, glaze, or otherwise.) parts of the surrounding environment start to interact with those orbitals. As they are more (or less) negative or move in closer (or are farther away), the surrounding environment starts to change the energy spacing of the orbitals.10,11
Even though they both contain a chromium impurity for color, emeralds are constructed from beryllium aluminum silicate (beryl) as opposed to corundum.