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Moonstone is a variety of the feldspar mineral orthoclase, characterized by its distinctive optical phenomenon known as adularescence. This Schiller effect, or 'moon-like glow,' appears as a billowy, bluish-white light that seems to glide across the surface of the gemstone when it is moved. The most prized moonstones exhibit a strong blue adularescence. Chemically, it is a potassium aluminum silicate, KAlSi3O8, belonging to the monoclinic crystal system. While often associated with orthoclase, the term 'moonstone' can also be applied to other feldspars (like albite or labradorite) that display adularescence, though orthoclase moonstone (adularia) is the classic and most common type.
How to Identify
- Color
- Typically colorless to white, but can also be 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 'schiller' or 'adularescence' effect.
- Crystal Form
- Usually found in massive or tabular forms. Crystals are typically monoclinic, often prismatic or blocky, but well-formed crystals are rare in gem quality material.
- Cleavage
- Perfect in two directions, intersecting at approximately 90 degrees (specifically 90 degrees and 90 degrees 30 minutes).
- Geological Environment
- Occurs in pegmatites, granites, syenites, and some metamorphic rocks. It is often found in hydrothermal veins and alpine-type fissures, particularly the adularia variety.
Key Facts
- Hardness: 6-6.5 on the Mohs scale.
- Specific Gravity: 2.56-2.59.
- Crystal System: Monoclinic.
- Color: Colorless, white, gray, brown, yellow, pink, green; with blue or white adularescence.
- Luster: Vitreous to pearly.
- Transparency: Transparent to translucent.
- Fracture: Uneven to conchoidal.
- Cleavage: Perfect in two directions (basal {001} and prismatic {010}) at approximately 90 degrees.
- Composition: Potassium aluminum silicate (KAlSi3O8).
Quick Check
- Color: Colorless to white, gray, brown, yellow, pink, green, with a characteristic blue or white adularescence.
- Luster: Vitreous to pearly.
- Streak: White.
Physical Characteristics
- Crystal Habit: Typically massive, tabular, or prismatic. Often found as cleavage fragments or anhedral grains in rocks. Well-formed crystals are rare in gem quality.
- Cleavage Type: Perfect in two directions, intersecting at 90 degrees and 90 degrees 30 minutes.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces.
Formation
Moonstone forms in igneous rocks (like pegmatites and granites) and metamorphic rocks (like gneisses and schists) under specific conditions that allow for the intergrowth of orthoclase and albite layers. The adularescence is caused by the scattering of light from these microscopic lamellar intergrowths of two different feldspar species (orthoclase and albite).
Usage
Primarily used as a gemstone in jewelry due to its unique adularescence. Also collected by mineral enthusiasts.
Age Distribution
Found in geological formations of various ages, typically associated with igneous and metamorphic processes.
Where to Find
Sri Lanka
Historically the primary source of high-quality blue moonstone, particularly from the Meetiyagoda mines.
India
Produces a significant amount of moonstone, often with a more milky or brownish body color and a less intense blue flash.
Myanmar (Burma)
Known for producing fine moonstone, sometimes with a strong blue adularescence.
Madagascar
A source of various feldspar varieties, including moonstone.
Tanzania
Produces moonstone, often with a yellowish or brownish tint.
Brazil
Some occurrences of moonstone have been reported.
United States (e.g., New Mexico, Virginia)
Minor occurrences of moonstone have been found in various states.
Finding Tips
Look for Adularescence
The most crucial identifying feature is the adularescence. Rotate the specimen under a light source to observe the characteristic floating, bluish-white sheen. This effect is best seen in diffused light rather than direct, harsh light.
Check for Cleavage
Feldspars, including moonstone, exhibit perfect cleavage in two directions at nearly right angles. This can be observed on broken surfaces.
Hardness Test (with caution)
Moonstone has a Mohs hardness of 6-6.5. It can scratch glass (hardness 5.5) but can be scratched by quartz (hardness 7). Use this test carefully on an inconspicuous area to avoid damaging a valuable specimen.
Examine for Inclusions
Moonstone can sometimes contain fine needle-like inclusions (often rutile) or centipede-like inclusions, which can enhance or interfere with the adularescence.
Distinguish from Imitations
Be aware of glass imitations or synthetic spinels that might mimic moonstone's appearance. These typically lack the natural adularescence and often show gas bubbles or swirl marks.
Similar Rocks
Labradorite
Labradorite ((Ca,Na)(Al,Si)2Si2O8)
Also known as: Spectrolite (a high-quality variety)
Rainbow Moonstone
Labradorite ((Ca,Na)(Al,Si)2Si2O8)
Also known as: White Labradorite
Opal
Hydrated Amorphous Silica (SiO2·nH2O)
Also known as: Precious Opal
Scientific Classification
- Mineral Class
- Silicates
- Group
- Feldspar Group (Potassium Feldspar Subgroup)
- Crystal System
- Monoclinic
- Chemical Formula
- KAlSi3O8
- Composition
- Potassium Aluminum Silicate
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