Corundum Rock Stars
The definition of a phenomenon as “an object or aspect known through the senses,” according to Webster’s dictionary, might explain why some gemstones are called phenomenal. Or it could be because these gems are “exceptional, unusual or abnormal.”
In reality, however, it is how the structure, inclusions and properties of these gemstones interact with light to return an unusual visual effect, or phenomenon, in the parlance of gemology, that make them phenomenal.
Regardless, we experience these gemstones as fascinating and recognize that they are, in fact, extraordinary.
Opal, labradorite and moonstone form in microscopic layers of material that are successively stacked upon each other, giving rise to phenomena based on these unique structures. When light interacts with the different structures of gem material it causes flashes of color.
Let’s see how their different structures interact with light properties.
Opal is made up of millions of tiny silica spheres stacked in layers and held together by a mixture of slightly different silica and water. When light interacts with the silica spheres, a diffraction of light causes flashes of spectral colors. These colors depend on the size of the silica spheres. Smaller spheres produce blue to green spectral colors and larger spheres produce red. This phenomenon is called play-of-color.
Labradorite is formed by alternating layers of two feldspars that have different chemical compositions. When light interacts with labradorite, the diffraction of light causes the phenomenon known as labradorescence. The perceived colors are dependent on the thickness of the alternating layers and their refractive indices.
Moonstone is composed of two different types of feldspars that vary in thickness and regularity. In moonstone, the light is scattered in many directions giving it the characteristic cloudy bluish-white light, known as adularescence.
Iridescence happens when light passes through a thin, transparent film that has a different refractive index from the surrounding material. The color patterns in soap bubbles or on a thin film of oil on water are examples of iridescence. Pearl nacre and mother-of-pearl can have just the right structure to display iridescent optical effects. Orient is a special term used to describe pearl iridescence.