Moonstone is a captivating gemstone belonging to the feldspar mineral group, cherished for its soft, glowing appearance. While the stone does not chemically change its fixed body color, it displays a unique, moving sheen that creates the illusion of a color shift. This visual phenomenon, which appears to float just beneath the surface, gives moonstone its ethereal, moonlit quality.
Adularescence: The Optical Phenomenon
The illusion of color change is caused by adularescence, an optical effect described as a soft, billowy light emanating from within the stone. This phenomenon results from moonstone’s microscopic internal structure. Moonstone is composed of two intergrown feldspar types, orthoclase and albite, which separate into extremely thin, stacked layers as the mineral cools.
When light enters the stone, it interacts with these alternating layers, which have slightly different refractive indices. The light waves scatter and interfere as they pass through these microstructures, reflecting a diffused, glowing sheen back to the observer. The thickness of these internal layers (lamellae) determines the sheen’s color; thicker layers scatter all wavelengths evenly, resulting in a whiter, milky sheen.
Conversely, thinner layers are more effective at scattering the shorter wavelengths of visible light, which correspond to blue. This differential scattering is similar to how the Earth’s atmosphere scatters sunlight to make the sky appear blue. Therefore, the most highly prized and intense effect is a vivid blue sheen. The light appears to have depth rather than reflecting off the surface, making the glow seem to move beneath the stone’s dome.
Base Colors and Mineral Varieties
While adularescence provides the floating, shifting light, the overall appearance of any moonstone is also determined by its fixed underlying body color. Moonstone’s base color can vary significantly, ranging from colorless and semi-transparent to translucent shades of white, gray, yellow, peach, or pink. The combination of this inherent body color and the floating adularescence dictates the stone’s final look.
The perception of color change is heightened in stones with a transparent or near-colorless body, as the blue adularescence stands out dramatically. A common source of confusion is “Rainbow Moonstone,” which is technically a variety of the feldspar labradorite. This variety displays a multi-colored flash called labradorescence, but it is widely marketed under the moonstone name due to its similar optical effect.
How Lighting and Viewing Angle Affect Perception
The visibility and intensity of the moonstone’s adularescence are highly dependent on external factors, specifically the light source and the angle of observation. Since the glow is an optical phenomenon, it exists only in the presence of light. Bright, direct light, such as sunlight or a focused LED spotlight, maximizes the scattering effect, making the sheen appear vivid and defined. Conversely, diffused or low light conditions diminish the effect, causing the stone to display only its fixed body color.
The viewing angle is equally important because the adularescence is strongly directional; the glow is only visible when the light strikes the stone’s internal layers at the correct orientation. This directional nature is why the sheen appears to “move” or “roll” across the surface as the stone or the observer shifts position. To maximize this effect, the stone is typically cut into a smooth, dome shape called a cabochon. If the stone is turned slightly, the scattering angle is lost, and the color disappears, completing the illusion of color change.