Moonstone is a deeply admired gemstone, prized for its unique glow and historical significance. Geologically, it belongs to the feldspar mineral group, specifically an orthoclase feldspar, though “rainbow moonstone” is technically a plagioclase feldspar. Identifying an authentic specimen requires careful observation beyond a simple glance at its color or shape. This guide provides practical methods and scientific details needed to confidently verify whether a stone is a genuine moonstone by focusing on its specific optical and physical traits.
Understanding Adularescence
The signature feature of genuine moonstone is a mesmerizing optical phenomenon known as adularescence, often described as a floating, billowy sheen. This effect appears to hover just beneath the surface of the gemstone, giving it a soft, internal luminescence. To properly observe adularescence, the stone should be examined under a single, direct light source and slowly rotated to observe the movement and depth of the light reflection.
Adularescence is caused by light scattering off minute, alternating layers of two different types of feldspar within the stone’s internal structure. These thin, microscopic layers of albite and orthoclase are intergrown, creating a lamellar structure. When light enters the stone, it diffracts and reflects from these layers, producing the characteristic glow, whose intensity is determined by the layers’ thickness and regularity.
The floating appearance of the sheen is crucial, as it distinguishes adularescence from simple surface reflections common on polished glass or quartz. When rotating a genuine moonstone, the light should shift and roll within the stone, not merely flash off the outer facet. Observing this internal depth is a primary test for confirming authenticity, as it is difficult to replicate precisely in manufactured materials.
The most highly valued specimens display a distinct blue adularescence, often called a “blue flash,” which is concentrated and appears to move across the dome of the stone. Lower-quality moonstones typically exhibit a white or silvery sheen, which is less defined and more diffuse. A synthetic or imitation stone often displays a surface shine rather than the deep, internal glow that defines true adularescence.
Secondary Physical Characteristics
Beyond the optical effect, moonstone possesses several static physical properties that aid in identification. Its hardness registers at approximately 6 on the Mohs scale, placing it in a relatively soft category for a gemstone. This moderate hardness means that moonstone can be scratched by harder materials like common quartz (Mohs 7).
A significant structural characteristic is its perfect cleavage in two distinct directions. Cleavage refers to the tendency of a crystalline mineral to break along specific planes of weakness. This internal structure makes the stone susceptible to chipping or fracturing along flat, parallel surfaces if struck sharply, which provides evidence of its feldspar composition.
Authentic moonstone typically ranges from semitransparent to translucent, meaning light passes through it, but images are often distorted. The body color is usually subtle, ranging from colorless to white, gray, brown, or a delicate peach hue. If a stone exhibits a strong, saturated body color alongside the optical effect, it should be viewed with suspicion, as vibrant color saturation is rare in natural orthoclase moonstone.
The specific gravity, or density, of moonstone is around 2.56 to 2.59, which is relatively low compared to many other gemstones. A simple tactile test can be performed by comparing the weight of a suspect stone to a known stone of similar size. If the unknown stone feels noticeably lighter or heavier than expected for its volume, its density may not align with genuine moonstone.
Common Look-Alikes and Fakes
One of the most frequent imitations is manufactured glass, often sold under trade names like Opalite. Glass simulants may possess an attractive sheen, but this effect is superficial and uniform, lacking the deep, internal movement of adularescence. Additionally, glass tends to feel noticeably warmer to the touch than natural stone, which typically draws heat away from the skin, feeling cool.
The term “Rainbow Moonstone” is often used commercially for a variety of plagioclase feldspar, which is technically labradorite. While related, this stone displays labradorescence, a broader spectral flash of many colors, rather than the simple blue or white sheen of classic orthoclase moonstone. True orthoclase moonstone exhibits a narrower, more focused schiller effect, while labradorite’s effect covers a wider color range.
Other minerals, such as Cat’s Eye Quartz, can sometimes exhibit a similar optical effect called chatoyancy or a flash. However, these will usually fail the other physical tests. Cat’s Eye Quartz, for example, is significantly harder (Mohs 7) than moonstone and possesses a different density. Confirming a stone’s identity requires the combination of the specific adularescence effect with the correct hardness and cleavage pattern.