Moonstone is a prized gemstone known for its characteristic glow that seems to float just beneath the surface, an effect that has captivated people for centuries. This unique visual appeal links the stone to the moon’s soft, ethereal light, giving the mineral its recognizable name. To understand this phenomenon, it is necessary to examine the mineral’s specific chemical makeup and internal structure.
The Chemical Composition and Mineral Family
Moonstone is fundamentally a member of the extensive feldspar mineral group, which constitutes a significant portion of the Earth’s crust. Specifically, it is a variety of the alkali feldspar orthoclase, but its special properties arise from an intimate intergrowth with a second mineral, albite. The general chemical formula for these components is a sodium-potassium aluminum silicate, represented as \((\text{Na},\text{K})\text{AlSi}_3\text{O}_8\).
The potassium-rich component is orthoclase, which has the formula \(\text{KAlSi}_3\text{O}_8\), while the sodium-rich component is albite, represented by \(\text{NaAlSi}_3\text{O}_8\). Moonstone exists as a solid solution series between these two end-members, meaning that at high temperatures, the potassium and sodium are mixed homogeneously within the crystal structure. This mixture is what defines the broader alkali feldspar family.
The mineral’s final composition is a blend where the potassium and sodium ions substitute for one another within the aluminum silicate structure. This mixture is not stable when the mineral cools down slowly over geological time. The slow separation of these two feldspar types is the foundation for the moonstone’s unique appearance, as the sodium and potassium atoms arrange themselves into distinct regions.
The Structural Cause of Adularescence
The soft, billowy sheen that appears to move across the moonstone’s surface is known as adularescence. This optical effect is not a reflection from the surface but rather a scattering of light that originates from within the stone’s internal structure. The process begins as the molten material containing the mixed feldspar cools very slowly deep within the Earth.
During this prolonged cooling, the originally homogeneous mixture of orthoclase and albite separates into alternating, ultra-thin layers. This geological process is called exsolution, where the two different feldspar compositions separate into parallel planes, or lamellae. These microscopic layers are typically only a few hundred nanometers thick, making them perfectly sized to interact with visible light.
When light enters the moonstone, it encounters these thin, alternating layers of orthoclase and albite, which have slightly different optical properties. The light waves are scattered in multiple directions as they pass through and reflect off the interfaces between these layers, creating a diffused glow. The resulting shimmer appears to float beneath the surface, and the color of the sheen—often blue or white—is determined by the thickness and spacing of these internal lamellae.
Sources and Common Color Variations
High-quality moonstone deposits are traditionally found in a few specific geographical locations. Historically, Sri Lanka has been renowned for producing some of the finest, most transparent moonstones that exhibit a striking blue adularescence. Other significant sources include India, Myanmar, and Madagascar, which also yield gems with varying degrees of clarity and sheen.
Moonstone can naturally occur in a range of body colors, including white, gray, peach, yellow, and occasionally green. A common variation seen in the market is “Rainbow Moonstone,” which is mineralogically distinct from true moonstone. Despite the trade name, Rainbow Moonstone is actually a variety of the plagioclase feldspar labradorite, which contains sodium and calcium instead of sodium and potassium.
This labradorite variety displays a multicolored flash of light, an effect called labradorescence. While both are members of the feldspar family, true moonstone is defined by its orthoclase-albite composition and soft, singular-toned adularescence, whereas Rainbow Moonstone is a different mineral with a multicolored sheen.