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Moonstone is a tectosilicate mineral, a variety of the feldspar group, known for its characteristic optical phenomenon called adularescence. This effect, a billowy, bluish-white to white light that appears to float just beneath the surface of the gemstone, resembles moonlight. The most prized moonstones exhibit a strong blue adularescence. While often associated with orthoclase (specifically adularia), moonstone can also be a variety of plagioclase feldspar (peristerite) or sanidine, all exhibiting the same optical effect due to their lamellar microstructure.
How to Identify
- Color
- Typically colorless to white, with shades of gray, brown, yellow, pink, or green. The most desirable moonstones exhibit a blue adularescence.
- Luster
- Vitreous to pearly.
- Texture
- Smooth when polished, often exhibiting a characteristic internal glow.
- Crystal Form
- Monoclinic (orthoclase variety) or Triclinic (plagioclase variety). Crystals are typically prismatic, tabular, or blocky, but moonstone is usually found in massive or granular forms, often cut as cabochons.
- Cleavage
- Perfect in two directions, intersecting at approximately 90 degrees (orthoclase) or slightly less than 90 degrees (plagioclase).
- Geological Environment
- Moonstone (adularia) is typically found in low-temperature hydrothermal veins and alpine-type fissures, often associated with granitic pegmatites, syenites, and other igneous rocks. Peristerite moonstone is found in pegmatites and metamorphic rocks.
Key Facts
- Hardness: 6.0 - 6.5 on the Mohs scale.
- Specific Gravity: 2.56 - 2.59 (Orthoclase); 2.55 - 2.75 (Plagioclase).
- Crystal System: Monoclinic (Orthoclase variety, Adularia) or Triclinic (Plagioclase variety, Peristerite).
- Color: Colorless, white, gray, brown, yellow, pink, green; with characteristic adularescence.
- Luster: Vitreous to pearly.
- Transparency: Transparent to translucent.
- Fracture: Uneven to conchoidal.
- Cleavage: Perfect in two directions, intersecting at approximately 90 degrees (orthoclase) or slightly less (plagioclase).
- Composition: Potassium aluminum silicate (KAlSi3O8) for orthoclase varieties; Sodium calcium aluminum silicate (Na,Ca)(Al,Si)AlSi2O8) for plagioclase varieties.
Quick Check
- Color: Colorless to white, gray, brown, yellow, pink, green; often with a bluish-white sheen (adularescence).
- Luster: Vitreous to pearly.
- Streak: White.
Physical Characteristics
- Crystal Habit: Prismatic, tabular, blocky, massive, granular. Often cut as cabochons to enhance adularescence.
- Cleavage Type: Perfect in two directions, {001} and {010}.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces.
Formation
Moonstone is primarily a variety of orthoclase feldspar, specifically adularia, or less commonly, a plagioclase feldspar (peristerite) or sanidine. It forms in igneous and metamorphic rocks. The characteristic adularescence (schiller) is caused by the diffraction of light within a micro-lamellar structure of alternating layers of orthoclase and albite (or other feldspar compositions). These layers are typically very thin (nanometer scale) and oriented in a specific way, allowing light to scatter and produce the characteristic bluish-white to white glow. This phenomenon is known as adularescence.
Usage
Moonstone is primarily used as a gemstone in jewelry due to its unique adularescence. It is often cut into cabochons to best display this optical effect. Historically, it has been used in various cultures for its perceived mystical properties.
Age Distribution
Moonstone forms in various geological settings and thus does not have a specific age distribution; its formation is tied to the age of the host rocks.
Where to Find
Sri Lanka
Renowned for producing high-quality blue moonstone, particularly from the Meetiyagoda mines.
India
Known for producing moonstones with a more milky or brownish hue, often with a strong adularescence.
Myanmar (Burma)
Produces moonstone, often with good clarity and adularescence.
Madagascar
Source of various qualities of moonstone, including some with blue adularescence.
Tanzania
Known for producing moonstone, sometimes with a yellowish tint.
Brazil
Occasional source of moonstone.
United States (e.g., New Mexico, Virginia)
Some localities produce moonstone, though generally not of gem quality.
Switzerland (Adula Alps)
The type locality for adularia, a variety of orthoclase that can exhibit adularescence.
Finding Tips
Look for Feldspar Deposits
Moonstone is a feldspar, so search in areas known for feldspar-rich igneous or metamorphic rocks, such as pegmatites, granites, and syenites, or in hydrothermal veins.
Examine for Adularescence
The key identifying feature is the adularescence. Rotate potential specimens under a light source to observe the characteristic floating, bluish-white to white glow. This effect is best seen in polished or tumbled pieces.
Check for Cleavage
Feldspars exhibit perfect cleavage in two directions. Look for flat, reflective surfaces where the mineral has broken along these planes.
Consider Associated Minerals
Moonstone can be found alongside other minerals common in pegmatites and hydrothermal veins, such as quartz, mica, and other feldspars.
Beware of Imitations
Glass or synthetic materials can imitate moonstone. Genuine moonstone will have a specific gravity and hardness consistent with feldspar, and the adularescence will appear to originate from within the stone, not just on the surface.
Similar Rocks
Labradorite
Labradorite (Plagioclase Feldspar)
Also known as: Spectrolite
Rainbow Moonstone
Labradorite (Plagioclase Feldspar)
Also known as: White Labradorite
Opal
Hydrated Amorphous Silica
Also known as: Precious Opal
Scientific Classification
- Mineral Class
- Tectosilicate
- Group
- Feldspar Group
- Crystal System
- Monoclinic (Orthoclase variety) or Triclinic (Plagioclase variety)
- Chemical Formula
- KAlSi3O8 (Orthoclase variety); (Na,Ca)(Al,Si)AlSi2O8 (Plagioclase variety)
- Composition
- Potassium aluminum silicate (for orthoclase moonstone); Sodium calcium aluminum silicate (for plagioclase moonstone).
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