Anorthite Value, Price, and Jewelry Information

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Custom square-cut anorthite with copper inclusions, 0.23 cts, 3.6 mm, Miyake-jima, Japan. © The Gem Trader. Used with permission.

The rarest plagioclase feldspar, anorthite is very rarely faceted.

Anorthite Information

Data Value
Name Anorthite
Is a Variety of Feldspar
Crystallography Triclinic
Crystallographic Forms
Refractive Index 1.577-1.590
Colors Colorless, white, gray, reddish, pale yellow
Hardness 6-6.5
Fracture Conchoidal to uneven
Specific Gravity 2.75-2.77
Birefringence 0.013
Cleavage Perfect 2 directions; imperfect 1 direction
Transparency Translucent to transparent
Optics α = 1.577; β = 1.585; γ = 1.590. Biaxial (-); 2V= 70°.
Optic Sign Biaxial -
Luminescence Present No
Luster Vitreous
Luminescence None
Formula CaAl2Si2O8
Pleochroism None
Etymology Anorthite is from the Greek words an plus orthos, meaning "not straight," because the crystal faces meet at an oblique angle.
Occurrence Mafic plutonic, volcanic, and metamorphic rocks. Sometimes makes up a distinctive rock known as anorthosite, which has been extensively studied.
anorthites - Italy

Anorthites on basalt, from the type locality, Mt. Somma-Vesuvius Complex, Napoli, Campania, Italy. © Rob Lavinsky, Used with permission.

What is Anorthite?

Anorthite is the most calcium-rich/sodium-poor plagioclase feldspar (90-100% Ca/10-0% Na). It forms a solid solution series with sodium-rich/calcium-poor albite (90-100% Na/10-0% Ca). The other plagioclase stones — oligoclase, andesine, labradorite, and bytownite — have intermediate compositions.

Although anorthites occur in many locations throughout the world, in many rock types and environments, it’s the rarest plagioclase. Its crystals are seldom transparent. As a result, anorthites cut into gemstones are an extremely rare sight.

Does Anorthite Make a Good Jewelry Stone?

Some plagioclases, like labradorite, make popular gemstones. Nevertheless, with a hardness of 6 to 6.5 and perfect cleavage in two directions and imperfect in another, anorthites are still susceptible to damage from scratches and blows against hard surfaces. As ring stones, anorthites would need protective settings. Use for earrings, pendants, and brooches would be more advisable.

Of course, you’re more likely to find anorthites in a mineral collection than a jewelry collection.

Are There Synthetic Anorthites?

Scientists have synthesized anorthite crystals for mineralogical research. However, there’s no known jewelry use for this material.

Where are Anorthites Found?

Notable sources include the following:

  • United States: Great Sitkin Island, Alaska; Pala, California; Italian Mountain, Colorado; Grass Valley, Nevada.
  • Finland; Greenland; India; Italy; Japan; Sicily; Sweden; United Kingdom.
anorthite crystals - Japan

Anorthites, partially coated with a film of blackish, glassy lava, Miyake-jima, Izu Archipelago, Tokyo Prefecture, Japan. © Rob Lavinsky, Used with permission.

Stone Sizes

Rare, faceted gems are always small. However, a locality on Great Sitkin Island, Alaska has yielded cut gems as large as 8 carats. This pale yellow anorthite may be the largest known.

How to Care for Anorthites

Due to their cleavage, avoid cleaning anorthites with mechanical cleaning systems, like steam and ultrasound. Clean them only with a soft brush, mild detergent, and warm water. Consult our gemstone jewelry cleaning guide for more recommendations.

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