Is moonstone a mineral?
No. Moonstone is a gem name, not an IMA species: GIA calls it orthoclase and albite in layers about 0.5 micron thick, hardness 6 to 6.5. Which feldspar varies.
No. Moonstone is not a mineral species but a gem name for feldspar showing adularescence. GIA describes it as two feldspar minerals, orthoclase and albite, in stacked layers about 0.5 micron thick, hardness 6 to 6.5, specific gravity 2.58. The September 2026 IMA list gives each of those minerals a row, and has none for moonstone.
In short
- A gem name for an effect. The GIA moonstone description says that to be called moonstone, a mineral’s actual identity is not as important as the beauty of its adularescence. A case-insensitive scan of every line of the September 2026 IMA-CNMNC Master List finds 0 containing moonstone. The name describes a glow; it does not point to one row of the list.
- The two-row answer. GIA says moonstone is composed of two feldspar minerals, orthoclase and albite, intergrown and then separated into stacked layers. The IMA list gives each its own row: orthoclase, K(AlSi3O8), approved, 1962 special procedure; albite, Na(AlSi3O8), grandfathered, 1815, Sweden. GIA’s 2025 review of phenomenal gems adds that the individual lamellae are always close to the end-member compositions. Two species in layers cannot be one species.
- Strict and loose, both from GIA. The 2025 Gems & Gemology review says only cryptoperthite, an alkali feldspar with nonperiodic submicron intergrowths of albite and orthoclase, can be called moonstone, and that the name has been haphazardly applied to peristerite and transparent labradorite. The GIA encyclopedia allows sanidine moonstones and records that Madagascan labradorite is known in the trade as rainbow moonstone. The table sets them side by side.
- Documented failure: peristerite. GIA’s review says peristerite is nearly always misidentified as moonstone because they look alike. It is sodic plagioclase, An2 to An16 in bulk, mostly low albite with minor oligoclase, and GIA says it is easily identified by polysynthetic Albite-twin lamellae and by the orientation of its iridescence.
- Adularia: the sources disagree. The International Gem Society says moonstones are sometimes called adularia, after Mt. Adular in Switzerland. GIA’s 2025 review calls the term adularescence misleading, because adularescence is not observed in adularia. Neither adularia nor moonstone has a row on the IMA list.
| Name on the label | What the source says it is | Feldspar branch | Composition as stated | IMA list, September 2026 | Source |
|---|---|---|---|---|---|
| Moonstone (broad) | A variety of orthoclase; orthoclase and albite in alternating layers | Alkali | KAlSi3O8 (facts panel) | No moonstone line; orthoclase A, 1962 s.p.; albite G, 1815 | GIA moonstone pages |
| Moonstone (strict) | Cryptoperthite: nonperiodic submicron albite-orthoclase intergrowth | Alkali | Spinodal type about Ab80Or20 to Ab40Or60; lamellae near the end members | No line contains perthite; albite and orthoclase rows | G&G, Summer 2025 |
| Sanidine moonstone | Moonstones containing sanidine, Grant County, New Mexico (IGS); sanidine can include adularescent gems called moonstones (GIA) | Alkali | K(AlSi3O8) (IMA) | Sanidine G, 1808, Germany | IGS; GIA |
| Rainbow moonstone | Labradorite, the calcium-rich plagioclase | Plagioclase | An50 to An70 (G&G 2025) | No labradorite line | G&G Spring 2024; IGS |
| Peristerite | Sodic plagioclase with exsolution textures; nearly always misidentified as moonstone | Plagioclase | An2 to An16 bulk; low albite near An0 with oligoclase near An20 | No peristerite line; albite G, 1815 | G&G, Summer 2025 |
| Adularia | IGS: a name moonstones are sometimes called; GIA: adularescence is not observed in adularia | Alkali, potassium-rich (GIA) | Not stated | No adularia line | IGS; G&G 2025 |
| Milky chalcedony; synthetic spinel | Simulants that usually look inferior and are easily spotted | Not feldspar | Not stated | Quartz SiO2 A, 1967 s.p.; spinel MgAl2O4 G (a synthetic is not a mineral) | IGS |
Is moonstone a mineral? No, it is a gem name for an effect
Moonstone is the gem name for feldspar that shows adularescence, a billowing sheen; it is not a mineral species. The GIA moonstone facts page calls moonstone a variety of orthoclase and gives its chemistry as KAlSi3O8, Mohs hardness 6.0 to 6.5, refractive index 1.518 to 1.526 and specific gravity 2.58. The same institute’s description says that, to be called moonstone, a mineral’s actual identity is not as important as the beauty of its adularescence. That is a definition by appearance, which is how gem names work and how species names do not.
What the IMA list records. The September 2026 IMA-CNMNC Master List was converted with pdftotext -layout and every line searched case-insensitively: 0 lines contain moonstone, adularia, peristerite, perthite or labradorite. The minerals inside moonstone do have rows: orthoclase, albite and sanidine. The clause-by-clause test of what makes something a mineral is worked on is quartz a mineral and crystal vs mineral, and is not repeated here.
Is my moonstone a mineral, then? It is made of minerals, feldspars, and it is natural; what it is not is one species. The honest label for a moonstone cabochon is feldspar, variety moonstone, or orthoclase with albite if the source supports it. Jewellery is named here only because that is what people ask about; this is not buying advice.
What is moonstone made of? The two-row answer
Moonstone is two minerals in layers, and the IMA list gives each its own row. The GIA description says moonstone is composed of two feldspar minerals, orthoclase and albite: at first intermingled, then, as the mineral cools, separated into stacked, alternating layers, and light scattering between those thin, flat layers produces adularescence. The facts page puts the layers at about 0.5 micron, similar to the size of a wavelength of light. The September 2026 list gives orthoclase K(AlSi3O8), status A, 1962 s.p., and albite Na(AlSi3O8), status G, 1815, Sweden.
Why it stays two. Jin, Renfro, Palke and Shigley, Gems & Gemology, Summer 2025 explains that the alkali feldspar series between albite and orthoclase has one large miscibility gap, and that the individual lamellae in moonstone are always close to the end-member compositions. Intermediate alkali feldspars of about Ab80Or20 to Ab40Or60 unmix by spinodal decomposition into alternating albite and orthoclase lamellae; more potassic ones grow thin, lens-shaped albite films in an orthoclase matrix, which give a white rather than blue sheen.
The two minerals on their own sheets. The Handbook gives orthoclase hardness 6 to 6.5, measured density 2.55 to 2.63, and records that it may exhibit opalescence or schiller iridescence; albite is 6 to 6.5 and 2.60 to 2.65, and may be chatoyant. The gem society’s moonstone range, 2.56 to 2.59, sits inside orthoclase’s. The species page is orthoclase. How moonstone is separated from labradorite on a refractometer is on is labradorite a moonstone.
Which feldspars are sold as moonstone? Strict and loose definitions
On GIA’s strict 2025 definition only one feldspar texture is moonstone; on its encyclopedia’s broad one, several feldspars are. The Gems & Gemology review says only cryptoperthite, an alkali feldspar with nonperiodic submicron intergrowths between albite and orthoclase, can produce that desaturated iridescence and be called moonstone, and that the name has been haphazardly applied to any light-coloured feldspar with a diffuse sheen, such as peristerite or transparent labradorite. The review sorts iridescent feldspars into three categories by bulk composition: labradorite, peristerite and moonstone.
The broad usage. The GIA description says sanidine is another feldspar that can include adularescent gems called moonstones, and the gem society places sanidine moonstones in Grant County, New Mexico. Labradorite from Madagascar with a multicoloured sheen is known in the trade as rainbow moonstone; GIA says it is actually a variety of labradorite rather than orthoclase, and the Gems & Gemology note on rainbow moonstone (Spring 2024) says it belongs to the plagioclase group and falls in the calcium-rich labradorite variety. That pairing has its own page, is labradorite a moonstone, with its own rule.
Documented failure: peristerite. GIA’s review says peristerite is nearly always misidentified as moonstone. It is a sodic plagioclase, An2 to An16 in bulk, mostly low albite with minor oligoclase, best known from Ontario, Canada, with mostly light blue iridescence. GIA says it is easily identified by polysynthetic Albite-twin lamellae and by the orientation of the iridescence, which, as in labradorite, is observable on the (010) cleavage. So a stone sold as moonstone that shows polysynthetic Albite-twin lamellae is, on GIA’s account, a candidate for peristerite rather than orthoclase-albite moonstone; is labradorite a feldspar sets out the plagioclase numbers.
When the answer changes: adularia, coatings and simulants
Adularia is where the two references part. The International Gem Society moonstone article says moonstones are sometimes called adularia after Mt. Adular in Switzerland, a famous source, and that the mountain lent its name to adularescence. GIA’s 2025 review calls the term adularescence misleading, as adularescence is not observed in adularia, a variety of potassium-rich feldspar. This page prints both and does not choose. Neither adularia nor moonstone has a row on the IMA list, so the species answer is the same either way.
Simulants and treatment, per the gem society. The IGS says moonstone has been simulated by milky chalcedony and certain types of synthetic spinel, which usually look inferior and are easily spotted, and that lab-created moonstones have not entered the market. It lists a dark coating on the back as an uncommon enhancement of adularescence, easily scratched and detected with magnification. Chalcedony is quartz; see chalcedony. Synthetic spinel is manufactured and so not a mineral at all.
The short rule. Moonstone tells you the effect; the mineral is the feldspar behind it. If the source names orthoclase with albite, write that; if it names labradorite or peristerite, the stone is a plagioclase sold under a moonstone name. The classification questions are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.