Labradorite
Labradorite is plagioclase feldspar, An50 to An70: hardness 6 to 6.5, density 2.68 to 2.72, not an IMA species. A 20 g piece reads 12.54 to 12.65 g in water.
Labradorite is a plagioclase feldspar, not an IMA species: its Handbook formula, Na0.5-0.3Ca0.5-0.7Al1.5-1.7Si2.5-2.3O8, is the band GIA defines as An50 to An70. It is triclinic, hardness 6 to 6.5, density 2.68 to 2.72, and commonly iridescent on {010}. The type locality is Pauls Island, Labrador, Canada.
In short
- A name for a composition band, not a species. A case-insensitive scan of every line of the September 2026 IMA-CNMNC Master List finds 0 lines containing labradorite, oligoclase, andesine or bytownite; albite and anorthite, the two ends of the series, have rows. The Handbook of Mineralogy keeps a labradorite sheet in the feldspar group, plagioclase series, and GIA’s 2025 review of phenomenal gems says the name without modification refers to any plagioclase between An50 and An70.
- The 0.11-gram window. At the sheet’s density of 2.68 to 2.72, a 20.00 g piece reads 12.54 to 12.65 g suspended in water, a window 0.11 g wide. The andesine sheet tops out at 2.68, reading 12.54 g, and the bytownite sheet starts at 2.72, reading 12.65 g. A balance confirms a plagioclase-weight feldspar; it cannot name labradorite. Quartz reads 12.45 g.
- Where the flash comes from. The sheet says labradorite commonly exhibits iridescence, especially on {010}. GIA’s 2025 review traces it to the Bøggild intergrowth: calcic lamellae of about 70 to 200 nm alternating with sodic lamellae of about 50 to 100 nm, where a combined thickness near 150 nm gives blue and over 250 nm gives red.
- Not every labradorite flashes. GIA puts the bulk composition of iridescent labradorites between An46 and An60, each give or take 2. That range covers only part of the An50 to An70 band, and its lower end sits below the An50 line GIA itself draws. The tests for a flashing stone are on how to tell if labradorite is real.
- Where. The Distribution field opens with Ford Harbour, Pauls Island, Labrador, then Lake St. John, Quebec; the Adirondack Mountains, New York; Sagebrush Flat, Lake Co., Oregon; abundant gem crystals in the Pinacate volcanic field, Sonora, Mexico; Ylämaa, near Lappeenranta, Finland; and the Langesundsfjord-Larvik-Tvedalen area, Norway.
| Name (Handbook sheet) | Ca in the sheet formula | An range | IMA row (September 2026) | Hardness | Density (measured) | 20.00 g piece in water | γ (sheet) |
|---|---|---|---|---|---|---|---|
| Albite | 0.0 to 0.1 | An0 to An10 | Na(AlSi3O8), G, 1815, Sweden | 6 to 6.5 | 2.60 to 2.65 | 12.31 to 12.45 g | 1.534 to 1.541 |
| Oligoclase | 0.1 to 0.3 | An10 to An30 | No row | 6 to 6.5 | 2.63 to 2.66 | 12.40 to 12.48 g | 1.542 to 1.552 |
| Andesine | 0.3 to 0.5 | An30 to An50 | No row | 6 to 6.5 | 2.66 to 2.68 | 12.48 to 12.54 g | 1.552 to 1.563 |
| Labradorite | 0.5 to 0.7 | An50 to An70 | No row | 6 to 6.5 | 2.68 to 2.72 | 12.54 to 12.65 g | 1.563 to 1.573 |
| Bytownite | 0.7 to 0.9 | An70 to An90 | No row | 6 to 6.5 | 2.72 to 2.74 | 12.65 to 12.70 g | 1.578 to 1.585 |
| Anorthite | 0.9 to 1.0 | An90 to An100 | Ca(Al2Si2O8), G, 1823, Italy | 6 to 6.5 | 2.74 to 2.76 | 12.70 to 12.75 g | 1.585 to 1.590 |
What is labradorite? A plagioclase feldspar between An50 and An70
Labradorite is the name for a plagioclase feldspar that is 50 to 70 per cent anorthite. The Handbook of Mineralogy labradorite sheet gives Na0.5-0.3Ca0.5-0.7Al1.5-1.7Si2.5-2.3O8, feldspar group, plagioclase series, with high- and low-temperature structural varieties recognised. Jin, Renfro, Palke and Shigley, Gems & Gemology, Summer 2025 writes that labradorite, without modification, refers to any plagioclase between An50 (50 mol% anorthite and 50 mol% albite) and An70. Read as An = 100 Ca / (Ca + Na), the sheet’s formula says exactly that. How a real analysis is checked against the band, worked end to end, is on is labradorite a feldspar.
What the sheet records. Triclinic; crystals typically thin, tabular along [010] and rhombic in section, to 4 cm; also cleavable, granular or massive. Twinning is common by the Albite, Pericline, Carlsbad, Baveno or Manebach laws. Cleavage is perfect on {001} and less perfect on {010}, the two intersecting at 90°, and distinct on {110}; fracture uneven to conchoidal; brittle. Hardness 6 to 6.5; measured density 2.68 to 2.72. Grey, brown, greenish, blue, yellow or colourless, white streak, vitreous lustre, pearly on cleavages. It occurs in mafic igneous rocks and anorthosites, more rarely in metamorphic rocks such as amphibolites, with olivine, pyroxenes, amphiboles and magnetite.
What the IMA list records. The September 2026 list was converted with pdftotext -layout and every line searched case-insensitively. No line contains labradorite, oligoclase, andesine, bytownite or the word plagioclase. Albite, Na(AlSi3O8), grandfathered from 1815 with Sweden as the country of first description, and anorthite, Ca(Al2Si2O8), grandfathered from 1823, Italy, have rows. So labradorite is a correct name for a feldspar of that composition, and not a species name. What a hand specimen can honestly be labelled is set out on how to identify feldspar.
Can you tell labradorite from andesine or bytownite? The 0.11-gram window
Not with a balance: labradorite’s weight in water shares both of its edges with its neighbours. Worked as 20 − 20/D on the measured densities in the table, a 20.00 g piece of labradorite reads 12.54 to 12.65 g in water. Andesine’s range ends at 12.54 g and bytownite’s begins at 12.65 g. The whole six-name ladder runs from 12.31 g for albite to 12.75 g for anorthite, 0.44 g end to end, with every neighbouring pair touching or overlapping. The method is on how to measure specific gravity at home; it confirms plagioclase weight and stops there.
Documented failure: a real labradorite that reads as bytownite. The International Gem Society labradorite article gives Oregon labradorite as Ab32An68, inside the An50 to An70 band, with S.G. 2.71 to 2.73. Worked, that is 12.62 to 12.67 g on a 20.00 g piece. The top of it, 12.67 g, lies inside the bytownite window of 12.65 to 12.70 g. The gem society’s general labradorite figure, 2.69 to 2.72, reads 12.57 to 12.65 g, the same as the moonstone and real-labradorite pages on this site.
Indices do slightly better, on a refractometer. On the sheets, labradorite’s γ of 1.563 to 1.573 meets andesine’s at 1.563 and stops 0.005 short of bytownite’s 1.578. That is an instrument reading, not a hand test, and hardness cannot help at all: all six sheets print 6 to 6.5. A composition, as An, is what names a plagioclase.
Why does labradorite flash? Labradorescence on {010}
Labradorite’s flash is interference from lamellae inside the crystal, seen on the {010} cleavage. The sheet records that it commonly exhibits iridescence, especially on {010}. GIA’s 2025 review calls the texture the Bøggild intergrowth: wider calcic lamellae of about 70 to 200 nm alternating with thinner sodic lamellae of about 50 to 100 nm, oriented so that the iridescence is observable on the (010) cleavage surface. A combined thickness of about 150 nm yields blue, more than 250 nm yields red, and a continuous spectrum lies between.
On GIA’s figures, only part of the band flashes. The same review says iridescent labradorites are found in plutonic rocks that cooled extremely slowly or underwent high-grade metamorphism, with bulk compositions between An46 and An60, each plus or minus 2, and that crystals can grow to several decimetres. A grey An65 labradorite with no flash is still labradorite; an An47 stone that flashes falls, on GIA’s own An50 line, in andesine’s range. GIA also defines labradorescence as the iridescent schiller of labradorite from plutonic rocks.
What this page does not repeat. The checks for whether a flashing stone is labradorite are on how to tell if labradorite is real; why a stone looks dead is on why is my labradorite turning black; and how the flash compares with moonstone’s is on is labradorite a moonstone. Whether moonstone is a mineral at all is on is moonstone a mineral.
Where labradorite is found: Labrador, Oregon, Sonora and Finland
The Distribution field calls labradorite widespread and names: Ford Harbour, Pauls Island, Labrador, Newfoundland; Lake St. John, Quebec, and elsewhere in Canada; in the USA, especially northern New York, forming the Adirondack Mountains; crystals from Sagebrush Flat, about 37 km north of Plush, Lake Co., Oregon; atop the San Marcos Mountains, San Diego Co., and in the western San Gabriel Mountains, Los Angeles Co., California; abundant gem crystals in the Pinacate volcanic field, Sonora, Mexico; Vesuvius and Mt. Etna, Italy; Ylämaa, near Lappeenranta, Finland; the Langesundsfjord-Larvik-Tvedalen area, Norway; and Surtsey Island, south of Iceland. Its name is for the type locality on Pauls Island.
Two references, two Labrador names. The sheet gives Ford Harbour, Pauls Island; the gem society says labradorite is best known from Nain, Labrador, with crystals up to 2 feet long but badly cracked. Both are in Labrador. GIA’s 2025 review names Canada, Finland and Madagascar for the best-quality iridescent labradorite, and says most Oregon sunstones are labradorites. The sheet’s Finnish locality is Ylämaa; the gem society says cut Finnish stones with very intense schiller are called spectrolite, without naming the place.
Its relatives on this site are the alkali feldspars orthoclase and microcline; the feldspar classification question is on is labradorite a feldspar. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.