What moonstone actually is
Moonstone is the iridescent variety of orthoclase feldspar, named for the floating-light effect, adularescence, caused by Bragg-like scattering of light off ~100-nanometer-scale alternating orthoclase and albite lamellae in a fine structural intergrowth[IMA][GIA]. The species is potassium aluminum silicate with the formula KAlSi₃O₈, crystallizing in the monoclinic system with a prismatic habit[Gem-A]. The historic mineralogical term adularia (named after Mt. Adula, Switzerland, where premium translucent feldspar was first described) has been replaced by "moonstone" as the modern gem-trade name[IGS].
A taxonomic distinction matters for buyers: moonstone is monoclinic orthoclase (alkali feldspar). Sunstone is triclinic plagioclase (calcium-sodium feldspar). Both are feldspars but distinct mineral species in different crystal systems[IMA][Gübelin]. The optical fingerprints labs use to identify moonstone are refractive index 1.518–1.526, specific gravity 2.55–2.66, birefringence 0.005–0.008, dispersion 0.012, biaxial negative optic sign, and a vitreous-to-silky luster (the silky sub-luster comes from the structural lamellae)[IGS][GIA]. Mohs hardness 6–6.5 plus perfect cleavage in two directions (basal + side pinacoid, characteristic of all feldspars) defines the species' working durability profile.
Color chemistry
Moonstone's body color is overwhelmingly white-to-translucent or peach. Body color reduces to White and Peach in the trade[GIA]. The trade nomenclature "blue moonstone," "rainbow moonstone," and "peach moonstone" refers to sheen color, not body color, and the distinction is consequential: sheen is a structural-iridescence phenomenon driven by the lamellar intergrowth, while body color is the underlying chromophore signal[IGS][Gem-A]. Peach body color comes from minor iron substitution in the feldspar lattice. The white-to-translucent body is the chromophore-poor norm.
The most prized commercial material pairs a colorless-to-translucent body with a vivid blue sheen. The body lets the floating light dominate the visual signal[GRS][SSEF]. White-body cabochons with weak adularescence anchor entry-tier commerce. Peach moonstone occupies a parallel mid-tier favored in bohemian and revival settings. Rainbow material (technically labradorite-feldspar, see Origin science) carries a separate market position from the orthoclase-true blue-sheen tier[IGS].
Origin science
Three origins anchor commercial supply: Sri Lanka (alluvial deposits in the Ratnapura district produce the historic premium "blue sheen" material since Victorian-era jewelry, the country's geology yields the thinnest orthoclase-albite lamellae and the most vivid blue adularescence. Contemporary supply is constrained), India (volume-supply origin for translucent white-body cabochons with weak-to-moderate adularescence, the dominant material in commercial mid-tier jewelry), and Madagascar (the source of "rainbow moonstone," technically a labradorite-feldspar variant, plagioclase intergrowth marketed under the moonstone trade name for the multi-color sheen across the visible spectrum)[USGS][IGS][Lotus]. Australia, Armenia, Mexico, Brazil, and the United States contribute additional rough at varying quality tiers[Gem-A].
Treatment landscape: the essentially-untreated species
Moonstone is essentially-untreated, reflecting the actual commercial state of the trade[GIA][LMHC]. The species' optical phenomena are intrinsic to the structural intergrowth. Adularescence cannot be enhanced by treatment, and the standard heat / irradiation / oiling toolkit either leaves the lamellar architecture unchanged or destroys it via thermal-shock fracture[CIBJO].
Some material is back-coated to enhance perceived sheen against dark backings, but that is a setting practice, not a gem treatment, and disclosure expectations follow CIBJO mounting-treatment conventions[CIBJO][SSEF]. An uncommon impregnation treatment with organic substances has been documented to improve adularescence and produce UV fluorescence, disclosed where detected[LMHC]. The dominant simulant landscape, leaded glass, opalite (a glass material), and occasionally labradorite or polymer-impregnated material sold under the moonstone name, is a separate disclosure issue from treatment and is the more common buyer-side risk in moonstone commerce[Gem-A].
Phenomena science
Blue sheen is the adularescence variant produced when the orthoclase-albite lamellae are thinnest and most uniform. The lamellar spacing preferentially scatters short-wavelength blue light, yielding the vivid sheen that defines premium Sri Lankan material[GIA][IGS]. The effect is centered on the cabochon dome and shifts position as the stone is tilted, the floating-light signature.
Rainbow is the adularescence variant produced when lamellae thickness varies across the crystal. The heterogeneous spacing scatters across the visible spectrum, yielding the multi-color sheen typical of Madagascar rough[Gem-A]. As noted above, "rainbow moonstone" is technically labradorite-feldspar (plagioclase intergrowth) rather than orthoclase, but the trade-nomenclature umbrella covers both under the moonstone designation[IGS].
Asterism in moonstone, star moonstone, is a rare cabochon-only variety produced by reflection from intersecting oriented inclusion sets, yielding a four-rayed star focused on the cabochon dome[GIA][IGS]. The phenomenon is structurally distinct from adularescence and demands careful cabochon orientation to center the star.
Chatoyancy, cat's-eye moonstone, is a similarly rare cabochon-only variety produced by reflection from oriented fibrous inclusions running parallel to a single crystallographic direction[Gem-A][IGS]. Like asterism, chatoyancy in moonstone is an oriented-inclusion phenomenon distinct from the species-defining structural iridescence.
The diagnostic inclusion signature for moonstone identification is the "centipede" or "caterpillar" inclusion, short pairs of cracks running parallel to the c-axis with shorter cracks emanating perpendicularly, resembling many-legged insects under magnification[Gem-A][GIA]. The presence of centipede inclusions confirms a stone as natural moonstone and rules out the most common simulants (leaded glass, opalite, polymer-impregnated material), which show swirls or bubbles instead[SSEF].
Species identity synthesized from the authoritative sources cited below.