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Rainbow Moonstone is a trade name for a variety of labradorite that exhibits a strong adularescence or labradorescence, often displaying a full spectrum of colors (blue, green, yellow, orange, pink, purple) against a white, colorless, or pale gray body. Chemically, it is a plagioclase feldspar, specifically an intermediate member of the albite-anorthite solid solution series, with a composition typically ranging from An50 to An70 (50-70% anorthite). The optical phenomenon is caused by the diffraction of light within microscopic lamellar intergrowths of different plagioclase compositions within the crystal structure.
How to Identify
- Color
- Body color typically white, colorless, or pale gray to light brown. The defining characteristic is the iridescent play of colors (labradorescence/adularescence) across the surface, which can include blue, green, yellow, orange, pink, and purple.
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Typically massive, granular, or in tabular crystals within host rock. When polished, it has a smooth, often slightly waxy feel.
- Crystal Form
- Usually found as anhedral to subhedral grains in igneous rocks. When well-formed, it can occur as tabular, prismatic crystals, often twinned (polysynthetic twinning visible as fine striations on cleavage surfaces).
- Cleavage
- Perfect in two directions, nearly at 90 degrees (86° and 94°), and good in a third direction.
- Geological Environment
- Occurs in mafic igneous rocks such as gabbro, basalt, anorthosite, and norite. Also found in some metamorphic rocks and as detrital grains in sediments derived from these rocks.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale.
- Specific Gravity: 2.68 to 2.72.
- Crystal System: Triclinic.
- Color: White, colorless, or pale gray body with iridescent flashes of blue, green, yellow, orange, pink, purple (labradorescence/adularescence).
- Luster: Vitreous to pearly.
- Transparency: Transparent to translucent.
- Fracture: Uneven to conchoidal.
- Cleavage: Perfect in two directions (pinacoidal), nearly at 90 degrees (86° and 94°).
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically an intermediate plagioclase feldspar with 50-70% anorthite (CaAl2Si2O8) and 30-50% albite (NaAlSi3O8).
Quick Check
- Color: White, colorless, or pale gray body with iridescent flashes of blue, green, yellow, orange, pink, purple.
- Luster: Vitreous to pearly.
- Streak: White.
Physical Characteristics
- Crystal Habit: Typically massive, granular, or in tabular to prismatic crystals. Often twinned (polysynthetic twinning).
- Cleavage Type: Perfect in two directions, nearly at right angles (86° and 94°).
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly.
Formation
Rainbow Moonstone, which is essentially a white or colorless variety of labradorite, forms primarily in mafic igneous rocks such as basalt, gabbro, and anorthosite. It crystallizes from magma rich in calcium and sodium, typically during the cooling and solidification of these rocks. The characteristic adularescence or labradorescence (schiller effect) is due to submicroscopic lamellar exsolution intergrowths of two feldspar phases (albite and anorthite) with slightly different compositions and refractive indices. Light scattering and interference within these lamellae produce the spectral colors.
Usage
Primarily used as a gemstone in jewelry due to its attractive adularescence/labradorescence. It is cut into cabochons, beads, and sometimes faceted. Also collected by mineral enthusiasts. Historically, feldspars have industrial uses in ceramics, glass, and as fillers, but 'Rainbow Moonstone' is specifically valued for its optical properties.
Age Distribution
Labradorite, as a component of igneous and metamorphic rocks, can be found in formations ranging from Precambrian to Cenozoic, depending on the specific geological event that formed the host rock.
Where to Find
Sri Lanka (Ceylon)
Historically a significant source of moonstone, including some varieties that might be termed 'Rainbow Moonstone' due to strong adularescence, though often orthoclase-based.
India
Known for producing significant quantities of 'Rainbow Moonstone' (white labradorite) with strong blue and multi-color flashes.
Madagascar
Another major source of 'Rainbow Moonstone' with good quality iridescence.
Myanmar (Burma)
Produces moonstone, some with iridescent qualities.
United States (various states)
Labradorite is found in several states, including Oregon (Oregon Sunstone, a variety of labradorite), but 'Rainbow Moonstone' specifically is less common as a primary source.
Canada (Labrador)
The original locality for labradorite, known for its strong labradorescence, though often with a darker body color than what is typically marketed as 'Rainbow Moonstone'.
Finland
Source of Spectrolite, a highly prized variety of labradorite with intense, full-spectrum labradorescence, often with a dark body color.
Finding Tips
Look for Mafic Igneous Rocks
Rainbow Moonstone (labradorite) is typically found in mafic igneous rocks like anorthosite, gabbro, and basalt. Search in areas known for these rock types.
Observe the Play of Color
The most distinctive feature is the labradorescence. Rotate potential specimens under a light source to observe flashes of spectral colors. This effect is best seen on polished or fractured surfaces.
Check for Cleavage
Feldspars exhibit perfect cleavage in two directions. Look for flat, reflective surfaces that meet at nearly right angles (approximately 86° and 94°).
Examine for Striations
Plagioclase feldspars, including labradorite, often show fine parallel striations (polysynthetic twinning) on cleavage surfaces, which can be a key identifier.
Consider Associated Minerals
Labradorite often occurs with other mafic minerals such as pyroxenes (augite), olivine, and magnetite.
Similar Rocks
Moonstone
Orthoclase or Albite (variety of Feldspar)
Also known as: Adularia
Labradorite
Labradorite (variety of Anorthite-rich Plagioclase Feldspar)
Also known as: Spectrolite (a specific variety)
Opal
Hydrated Amorphous Silica (SiO2·nH2O)
Also known as: Precious Opal, Common Opal
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Feldspar Group (Plagioclase Series)
- Crystal System
- Triclinic
- Chemical Formula
- (Ca,Na)(Al,Si)AlSi2O8
- Composition
- Solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with Rainbow Moonstone (labradorite) typically having a composition of An50-An70.
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