The gemstone known as Moonstone has captivated observers for centuries with its luminous appearance. Its name is derived from the distinct, gentle light that seems to pulse and shift across its surface, reminiscent of moonlight glowing on water. This unique visual effect gives the stone an ethereal quality. Moonstone can be found in various body colors, including white, peach, gray, and blue, but the soft, billowy sheen defines its beauty.
The Moonstone Mineral Family
Moonstone is a mineral, not a rock (which is an aggregate of minerals). It is a gem variety found within the massive and widespread Feldspar group, which accounts for over half of the Earth’s crustal material. The most common variety of gem-quality Moonstone is primarily a form of the mineral orthoclase, a potassium aluminum silicate with the chemical formula (K,Na)AlSi3O8. The presence of sodium and potassium allows for the distinctive structure that creates its visual appeal.
The stone is relatively durable for jewelry, registering a hardness of 6.0 to 6.5 on the Mohs scale. While the term Moonstone is sometimes applied to plagioclase feldspar varieties like albite or labradorite, the classic, most prized Moonstone remains the potassium-rich orthoclase mineral. Although most common forms are translucent, the finest examples possess a high degree of transparency and a strong color sheen.
The Internal Structure That Creates Adularescence
The floating light that characterizes Moonstone is an optical phenomenon known as adularescence, resulting from light scattering within the stone’s unique internal structure. During the mineral’s cooling process, two different feldspar types, orthoclase and albite, separate into microscopic, alternating layers. This process, called exsolution, involves the minerals unmixing from a high-temperature solid solution into fine layers, or lamellae, upon cooling.
When light enters the Moonstone, it interacts with these ultra-thin internal layers, which are often only a few hundred nanometers thick. This interaction causes the light to scatter, producing a soft, diffused glow that appears to billow just beneath the surface. The color of the sheen (blue, white, or silvery) depends directly on the thickness of these alternating layers; thinner layers produce the more sought-after blue adularescence.
Geological Formation and Global Sources
The formation of Moonstone requires the cooling of magma rich in specific chemical components. It typically forms within igneous rocks, particularly in pegmatites, which are coarse-grained formations created by the last vestiges of a magma chamber. The slow cooling rate in these environments allows the orthoclase and albite components to separate into the layered structure required for adularescence. Moonstone is also found in granite, an intrusive igneous rock, or in certain metamorphic rocks where feldspar minerals have recrystallized.
The distribution of this gemstone is global, with several regions providing commercially significant deposits. Historically, the finest and most transparent specimens with a vivid blue sheen came from Myanmar, but high-quality material is now rare. Today, the largest source of commercial Moonstone is Sri Lanka, where it is often mined from alluvial deposits. Significant sources also include India, known for a variety sometimes called “rainbow moonstone,” as well as Madagascar, the United States, and Australia.