Is labradorite a moonstone
Not strictly. GIA calls moonstone orthoclase layered with albite; labradorite is a calcic plagioclase at density 2.68 to 2.72. Rainbow moonstone is labradorite.
Not strictly. GIA calls moonstone a variety of orthoclase, an alkali feldspar layered with albite, hardness 6 to 6.5. Labradorite is a calcium-rich plagioclase feldspar with a measured density of 2.68 to 2.72, against orthoclase's 2.55 to 2.63. The overlap is rainbow moonstone, which GIA's Gems & Gemology and the International Gem Society both place in labradorite.
In short
- Two feldspars, two branches. GIA calls moonstone a variety of orthoclase, whose sheen comes from alternating layers of orthoclase and albite; GIA gives it KAlSi3O8, hardness 6.0 to 6.5 and specific gravity 2.58. Labradorite is in the plagioclase series: the Handbook of Mineralogy prints it between Na0.5Ca0.5 and Na0.3Ca0.7.
- Rainbow moonstone is labradorite. A Gems & Gemology note (Spring 2024) says rainbow moonstone belongs to the plagioclase group and falls into the calcium-rich labradorite variety. The International Gem Society says such stones are technically labradorite, and that the trade has generally accepted the name.
- The colour-count rule. Count the colours in the flash. Blue with green, gold or orange is labradorescence, and the stone is labradorite whatever the label says. A single blue or white sheen does not decide it, because GIA says labradorite and sanidine can show adularescence too.
- A refractometer decides a loose stone; hardness never does. Moonstone reads 1.518 to 1.526 (GIA) and labradorite 1.560 to 1.572 (IGS). Both are hardness 6 to 6.5. Density works on a clean loose piece, but the gap is narrow.
- Neither name is a species on the IMA list. Orthoclase, albite and sanidine have rows on the September 2026 list; labradorite and moonstone do not. The site's how to identify feldspar page sets out what a hand specimen can be labelled.
| Criterion | Moonstone (orthoclase with albite layers) | Labradorite | Does it choose? |
|---|---|---|---|
| Feldspar branch | Alkali feldspar: orthoclase (GIA) | Plagioclase series (Handbook); calcium-rich (GIA) | Only by analysis |
| Formula | KAlSi3O8 (GIA) | Na0.5-0.3Ca0.5-0.7Al1.5-1.7Si2.5-2.3O8 | Only by analysis |
| Name of the optical effect | Adularescence | Labradorescence, schiller | - |
| Colours in the flash | Billowing light; the finest a mobile, electric blue shimmer (GIA) | Blue, green, blue-green, gold, yellow, purple (IGS) | Yes, when two or more colours show |
| Refractive index | 1.518 to 1.526 (GIA) | 1.560 to 1.572 (IGS); α 1.555 to 1.565, γ 1.563 to 1.573 (Handbook) | Yes: about 0.03 apart |
| Hardness (Mohs) | 6.0 to 6.5 (GIA); orthoclase 6 to 6.5 | 6 to 6.5 | No |
| Density | 2.58 (GIA); orthoclase 2.55 to 2.63 | 2.68 to 2.72; IGS 2.69 to 2.72 | Weakly: 0.05 at the closest |
| 20.00 g loose piece, weight in water | 12.25 g at 2.58; 12.16 to 12.40 g for orthoclase | 12.54 to 12.65 g | Only on a 0.01 g scale: 0.14 g at the closest |
| Twinning on the sheet | Orthoclase sheet: Carlsbad, Baveno or Manebach; simple, contact or penetration | Albite, Pericline, Carlsbad, Baveno or Manebach; IGS lists repeated twinning | Weakly: fine parallel lines point to plagioclase |
| IMA list, September 2026 | Orthoclase: approved, 1962 s.p. | No row | - |
Is labradorite a moonstone? Two feldspars, and where the names meet
Labradorite and moonstone are both feldspars, but not the same one. The GIA moonstone description calls moonstone a variety of the feldspar-group mineral orthoclase. During formation, orthoclase and albite separate into alternating layers, and light scattered between them produces adularescence. The same page says other feldspars, labradorite and sanidine among them, can also show adularescence. The Handbook of Mineralogy prints labradorite in the feldspar group, plagioclase series, at Na0.5-0.3Ca0.5-0.7Al1.5-1.7Si2.5-2.3O8, the calcium-rich part of the series. Its name is for the type locality on Pauls Island, off the coast of Labrador.
Where the names meet: rainbow moonstone. The GIA Gems & Gemology note on rainbow moonstone (Spring 2024) says rainbow moonstone, which shows flashes of various colours that can include blue, belongs to the plagioclase group and falls into the calcium-rich labradorite variety. The International Gem Society moonstone article says such stones are technically a variety of labradorite, not moonstone, and that the trade has generally accepted the name. So labradorite is sold as a moonstone; it is not an orthoclase moonstone.
The colour-count rule: what the flash can and cannot tell you
Several colours in the flash mean labradorite; one colour does not decide. The International Gem Society labradorite article gives labradorite's schiller colours as blue, green, blue-green, gold, yellow and purple. The IGS moonstone article calls a multicoloured schiller, blue with green or orange, labradorescence, and says it is distinct from adularescence. GIA's finest moonstone shows a mobile, electric blue shimmer.
The rule, stated once: tilt the stone under one light and count the colours. Two or more spectral colours is labradorescence: call it labradorite. A single blue or white sheen is undecided. The IGS labradorite article records a labradorite moonstone from Madagascar with a blue sheen, and GIA says labradorite can show adularescence, so a single sheen is not proof of orthoclase. A single-sheen stone needs the refractometer.
What the Handbook sheets record. Of the six feldspar sheets in this site's 176-sheet reference set, three note an optical effect in the Color field. Labradorite commonly exhibits iridescence, especially on {010}; orthoclase may exhibit opalescence or schiller iridescence; albite may be chatoyant. Anorthite, microcline and sanidine note none. The word moonstone appears on none of the 176, and iridesc on 11. Method: pdftotext -layout text searched case-insensitively.
Refractive index and density: the numbers that separate a loose stone
Refractive index separates them by about 0.03, and density by less than a fifth of a gram on a 20 g piece. GIA gives moonstone 1.518 to 1.526; the IGS gives labradorite 1.560 to 1.572, and the Handbook's labradorite indices run from α 1.555 to γ 1.573. The orthoclase sheet's γ tops out at 1.525. On a gem refractometer that gap is decisive.
Density is weaker. Orthoclase's measured density is 2.55 to 2.63, and GIA gives moonstone 2.58; labradorite's is 2.68 to 2.72. A clean, loose 20.00 g piece weighs 12.16 to 12.40 g in water as orthoclase and 12.54 to 12.65 g as labradorite, 0.14 g apart at the closest. Albite, the other layer in moonstone, reaches 2.65 on its sheet, which narrows the gap further. Use a 0.01 g scale, and do not try it on a set stone. The method is on how to measure specific gravity at home.
Hardness cannot choose. Both are 6 to 6.5 on their sheets, and GIA gives moonstone 6.0 to 6.5.
Who each answer is wrong for
Who the answer moonstone is wrong for: anyone holding a grey or dark stone whose flash shows blue with green, gold or orange. That is labradorite, and the trade's rainbow moonstone is the same material. Who the answer labradorite is wrong for: anyone holding a colourless to white stone with a single billowing sheen that reads near 1.52 on a refractometer. Who neither suits: a single-sheen stone that cannot be tested, such as a bead in a bracelet. It is a feldspar with a sheen, and the honest label stops there.
The feldspars themselves are on the orthoclase and microcline species pages, and twinning is explained on mineral twinning explained. The guides are indexed at the collecting guides. Jewellery is named here only because that is what people ask about; this is not buying advice. Nothing here is offered for sale; the wanted list is the only commercial route.