Moonstones are not a creation of myth or fantasy but are indeed real, naturally occurring mineral specimens prized in jewelry and geology. This captivating gem is a variety of the feldspar mineral group, which is one of the most abundant mineral families on Earth. The stone is famous for its unique visual property: a milky or bluish internal glow that appears to float just beneath the surface. This luminous effect defines the moonstone and elevates it from a common mineral to a celebrated gemstone.
The Mineral Family and Chemical Makeup
Moonstone is a member of the feldspar family, specifically an alkali feldspar like orthoclase, sanidine, or microcline. Feldspars are tectosilicate minerals that make up over half of the Earth’s rocky crust. The chemical formula for common moonstone varieties is sodium potassium aluminum silicate, often written as (Na,K)AlSi3O8.
The stone’s structure results from a geological process called exsolution, which occurs as the mineral cools slowly from high temperatures. During cooling, two types of feldspar—orthoclase (potassium-rich) and albite (sodium-rich)—separate from a single homogeneous mineral. Instead of completely mixing, they form extremely thin, alternating layers, or lamellae, stacked within the internal structure. These layers are microscopic and represent the beginning of a process where the two minerals attempt to separate into distinct phases. This fine, layered structure is the physical foundation that allows the stone to display its signature optical effect.
The Phenomenon of Adularescence
The distinctive, floating light that seems to billow across a moonstone’s surface is known as adularescence. This term describes the bluish-white schiller, or sheen, that appears to move as the stone is tilted or the light source shifts. This is the most important quality factor for the gemstone, giving it an ethereal appearance often compared to moonlight.
The scientific mechanism behind adularescence is the scattering and interference of light interacting with the stone’s internal structure. Light penetrates the moonstone and hits the ultra-thin, stacked layers of albite and orthoclase feldspar. Since the layer thickness is comparable to the wavelength of visible light (around 0.5 microns), the light waves are scattered in multiple directions.
This scattering effect is known as the Tyndall effect, which creates the diffuse, soft glow. Light waves scattered and reflected back to the viewer interfere with one another. The resulting color of the sheen (blue, white, or multicolored) is determined by the specific thickness of the alternating mineral layers.
How to Identify Authentic Moonstone
Identifying a genuine moonstone requires paying close attention to its unique physical and optical properties, as many simulants and synthetics are present in the market. The most telling characteristic is the quality and movement of the adularescence. A real moonstone’s glow should appear to roll or glide across the entire dome of a cabochon-cut stone when rotated, rather than remaining fixed.
Fake stones, particularly those made from glass, often have a static, flat sheen that lacks this dynamic, billowy movement. A genuine moonstone will also feel cool to the touch and take a moment to warm up in your hand. In contrast, glass or plastic imitations feel lighter and warm much faster.
For gemological purposes, moonstone has a Mohs hardness of 6.0 to 6.5, which is relatively soft, making it susceptible to scratching by harder materials like quartz. The specific gravity, or density, of moonstone typically falls between 2.54 and 2.66.
Observing the stone under magnification can reveal natural inclusions, such as tiny internal fissures or stress cracks often described as “centipedes,” which are characteristic of natural formation. Flawless, highly transparent moonstones with an intense blue glow are rare and command the highest value. However, a slight cloudiness or minor inclusions are common and often a sign of authenticity.