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Rainbow Moonstone is a captivating gemstone known for its adularescence, a Schiller effect that creates a milky, bluish, or multicolored sheen across its surface, resembling moonlight. Despite its common name, it is mineralogically a variety of labradorite, a plagioclase feldspar, rather than orthoclase moonstone. Its base color is typically white to colorless, often with a translucent to semi-transparent appearance, allowing the vibrant play of color to be observed. The 'rainbow' effect refers to the flashes of blue, green, yellow, and sometimes pink or purple that can be seen as the stone is rotated.
How to Identify
- Color
- Typically white to colorless, translucent to semi-transparent, with a characteristic iridescent play of colors (blue, green, yellow, etc.) when light interacts with its internal structure.
- Luster
- Vitreous to pearly.
- Texture
- Smooth when polished (cabochon cut). Unpolished surfaces may show typical feldspar cleavage planes.
- Crystal Form
- Usually found as anhedral to subhedral masses or cleavable fragments. When euhedral, it forms tabular or prismatic crystals, though these are rare in gem quality.
- Cleavage
- Perfect in two directions, nearly at right angles (approximately 90 degrees for orthoclase, 86 degrees for plagioclase).
- Geological Environment
- Found in various igneous rocks (e.g., anorthosites, gabbros, basalts, syenites, granites, pegmatites) and some metamorphic rocks. It can also occur in hydrothermal veins.
Key Facts
- Hardness: 6 - 6.5 (Mohs scale)
- Specific Gravity: 2.56 - 2.62
- Crystal System: Monoclinic (for orthoclase moonstone) or Triclinic (for plagioclase/labradorite rainbow moonstone)
- Color: Colorless to white, with adularescence/labradorescence displaying blue, green, yellow, and other spectral colors.
- Luster: Vitreous to pearly
- Transparency: Translucent to semi-transparent
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions, nearly at 90 degrees (orthoclase) or 86 degrees (plagioclase)
- Composition: Potassium aluminum silicate (KAlSi3O8) for true moonstone (orthoclase variety) or Sodium Calcium Aluminum Silicate ((Na,Ca)(Al,Si)AlSi2O8) for Rainbow Moonstone (labradorite variety).
Quick Check
- Color: White to colorless with iridescent flashes of blue, green, yellow, etc.
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, granular, lamellar, or tabular to prismatic crystals (rarely euhedral).
- 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
Rainbow Moonstone, despite its common name, is not a true orthoclase moonstone but rather a variety of plagioclase feldspar, specifically labradorite. It forms in igneous rocks (like basalt, gabbro, anorthosite) and some metamorphic rocks. The characteristic adularescence (or labradorescence in this case) is caused by the diffraction of light within microscopic lamellar intergrowths of two different feldspar phases (albite and orthoclase for true moonstone, or albite and labradorite for rainbow moonstone/labradorite). These lamellae are typically exsolution lamellae, forming as a solid solution cools and separates into distinct mineral phases.
Usage
Primarily used as a gemstone in jewelry due to its attractive adularescence/labradorescence. It is cut into cabochons to best display this optical effect. Also collected by mineral enthusiasts.
Age Distribution
Varies widely depending on the geological formation; can be found in rocks ranging from Precambrian to Cenozoic.
Where to Find
Sri Lanka
Historically a primary source for high-quality moonstone (orthoclase variety), but also produces some material with rainbow sheen.
India
Significant source of rainbow moonstone, particularly from the Odisha region.
Madagascar
Known for producing various feldspar varieties, including those exhibiting adularescence/labradorescence.
Myanmar (Burma)
Produces moonstone, including some with a rainbow effect.
Canada (Labrador)
The namesake locality for labradorite, which is mineralogically identical to rainbow moonstone. Produces material with strong labradorescence.
Norway
Another source of labradorite with strong optical effects.
Finding Tips
Look for Feldspar Deposits
Rainbow moonstone, being a feldspar, will be found in geological environments rich in feldspars, such as pegmatites, granites, syenites, and anorthosites. Look for areas with exposed igneous or metamorphic rocks.
Examine for Adularescence/Labradorescence
The key identifying feature is the optical phenomenon. Rotate potential specimens under a strong light source to observe the characteristic blue, white, or rainbow sheen. This effect is best seen on smooth, fractured, or polished surfaces.
Check for Cleavage
Feldspars exhibit distinct cleavage. Look for two cleavage planes intersecting at nearly 90 degrees (for orthoclase) or slightly less (for plagioclase). This can help distinguish it from quartz or opal.
Hardness Test (Carefully)
With a Mohs hardness of 6-6.5, it can scratch glass but can be scratched by quartz. Use this as a supplementary test, being careful not to damage valuable specimens.
Consult Geological Maps
Research local geology and geological maps to identify areas known for feldspar-rich igneous or metamorphic intrusions.
Similar Rocks
Moonstone (Orthoclase)
Orthoclase (KAlSi3O8)
Also known as: Adularia, Hecatolite
Labradorite
Labradorite ((Ca,Na)(Al,Si)AlSi2O8)
Also known as: Spectrolite (a high-quality variety)
Opal
Opal (SiO2·nH2O)
Also known as: Precious Opal, Common Opal
Scientific Classification
- Mineral Class
- Silicates
- Group
- Feldspar Group (specifically, Orthoclase or Plagioclase series)
- Crystal System
- Monoclinic (for orthoclase moonstone) or Triclinic (for plagioclase/labradorite rainbow moonstone)
- Chemical Formula
- KAlSi3O8 (for orthoclase moonstone) or (Na,Ca)(Al,Si)AlSi2O8 (for labradorite/rainbow moonstone)
- Composition
- Potassium aluminum silicate (orthoclase) or Sodium calcium aluminum silicate (labradorite). The adularescence/labradorescence is due to microscopic exsolution lamellae of different feldspar compositions.
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