Moonstone is a unique gemstone variety of the mineral feldspar. It is primarily sought after not for its intrinsic body color, but for a captivating internal light display that appears to float just beneath its surface. This phenomenon gives the stone a soft, glowing appearance, historically associating it with the light of the moon. This visual effect is the defining characteristic that determines its appearance.
The Optical Phenomenon of Moonstone
The characteristic shimmer of moonstone is an optical effect called adularescence, also known as schiller or sheen. This glow appears to emanate from within the stone itself, resembling moonlight on water. Adularescence occurs because of the stone’s unique internal structure, consisting of microscopic, alternating layers of two different feldspar minerals (Orthoclase and Albite). When light enters, it is scattered and diffracted by these ultra-thin, layered structures (lamellae), producing the billowy, diffused light that moves across the stone. The specific color of the sheen, whether silvery-white or blue, depends on the thickness of these internal layers; thinner layers produce the more intense blue light.
The Base Body Colors of Classic Moonstone
Classic moonstone is mineralogically defined as an Orthoclase feldspar, and its base color provides the canvas for the adularescence. The most common base colors range from nearly colorless and transparent to translucent white, often appearing milky. Beyond these neutral tones, moonstone naturally occurs in a variety of soft pastel shades, including smoky gray or brown, soft peach or pink, and pale yellow or light green varieties. For instance, a stone with a smoky gray body might display a silvery or white sheen.
Understanding Rainbow Moonstone
A common source of confusion is the commercial term “Rainbow Moonstone,” which is not a true moonstone variety. Mineralogically, this material is a type of Plagioclase Feldspar, specifically Labradorite. While it belongs to the larger feldspar group, its optical effect is called labradorescence, which is distinctly different from adularescence. Labradorescence produces a full spectrum of colors, with vivid flashes of blue, green, yellow, and purple across the stone. The base body color is typically a white or colorless translucent material, often characterized by visible dark, hair-like inclusions.
Color, Clarity, and Rarity
The color and quality of the internal light display are the primary factors influencing a moonstone’s value. The most desirable and rarest material is characterized by a strong, vivid blue adularescence, often called “blue sheen.” This effect must appear over a colorless and transparent body, with the blue sheen floating sharply across the dome. Stones exhibiting a white or silvery sheen are more common and less valued than the blue-sheen variety. Clarity is also a determining factor, as eye-visible inclusions or an opaque, cloudy appearance reduces the stone’s desirability and price.