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Moonstone is a feldspar mineral known for its characteristic adularescence, a shimmering optical phenomenon resembling moonlight. While the term 'moonstone' is most commonly associated with orthoclase feldspar (specifically the variety adularia) with intergrown albite, it can also refer to plagioclase feldspars (e.g., albite-oligoclase) exhibiting a similar effect, often termed 'peristerite'. The adularescence is caused by the scattering of light from microscopic, alternating layers of different feldspar compositions within the mineral structure. The color of the sheen can range from blue to white, sometimes with a rainbow effect. The body color of moonstone can be colorless, white, gray, peach, or brown.
How to Identify
- Color
- Typically colorless, white, gray, peach, or brown body color, with a characteristic bluish-white to silvery sheen (adularescence).
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Smooth, often polished when cut as a gemstone. Can appear somewhat translucent to opaque.
- Crystal Form
- Adularia (orthoclase moonstone) typically forms in prismatic or tabular crystals, often twinned. Plagioclase moonstones can also be tabular. Often found as massive or granular aggregates.
- Cleavage
- Perfect cleavage in two directions at nearly 90 degrees (orthoclase) or slightly less than 90 degrees (plagioclase).
- Geological Environment
- Adularia moonstone is found in low-temperature hydrothermal veins, alpine-type fissures, and some pegmatites. Plagioclase moonstone (peristerite) is found in pegmatites and other igneous rocks.
Key Facts
- Hardness: 6-6.5 on the Mohs scale.
- Specific Gravity: 2.56-2.59 (Orthoclase); 2.61-2.62 (Plagioclase).
- Crystal System: Monoclinic (Orthoclase/Adularia); Triclinic (Plagioclase).
- Color: Colorless, white, gray, peach, brown, with a characteristic bluish-white to silvery adularescence.
- Luster: Vitreous to pearly.
- Transparency: Transparent to translucent.
- Fracture: Uneven to conchoidal.
- Cleavage: Perfect in two directions, nearly at 90 degrees (orthoclase) or slightly less (plagioclase).
- Composition: Potassium aluminum silicate (KAlSi3O8) for orthoclase/adularia moonstone; Sodium calcium aluminum silicate ((Na,Ca)(Al,Si)4O8) for plagioclase moonstone.
Quick Check
- Color: White, gray, peach, or colorless body with a blue to white sheen.
- Luster: Vitreous to pearly.
- Streak: White.
Physical Characteristics
- Crystal Habit: Prismatic, tabular, massive, granular.
- Cleavage Type: Perfect in two directions, nearly perpendicular.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly.
Formation
Moonstone forms primarily in igneous and metamorphic rocks. The adularescence (schiller) characteristic of moonstone is due to the intergrowth of two different feldspar species, typically orthoclase and albite, or two different plagioclase feldspars (e.g., albite and oligoclase) in very thin, alternating layers (lamellae). When light enters the stone, it scatters from these layers, producing the characteristic bluish-white to silvery sheen. This phenomenon is known as adularescence. Adularia, a low-temperature monoclinic polymorph of orthoclase, often forms in alpine veins and low-temperature hydrothermal deposits. Plagioclase moonstones (peristerite) form in pegmatites and other igneous environments.
Usage
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
Varies widely depending on the geological formation; can be found in rocks ranging from Precambrian to Cenozoic.
Where to Find
Sri Lanka
Renowned for producing high-quality blue moonstone, particularly from the Meetiyagoda mines.
India
Known for producing 'rainbow moonstone' (which is actually a variety of labradorite) and peach/gray moonstone.
Myanmar (Burma)
Historically a source of fine moonstone.
Madagascar
Produces various colors of moonstone, including white and peach.
Tanzania
A source of moonstone, often with a good adularescence.
United States (e.g., New Mexico, Virginia)
Localities where feldspar exhibiting adularescence can be found, though often not of gem quality.
Brazil
Produces some moonstone, often in pegmatite deposits.
Finding Tips
Look for Feldspar Deposits
Moonstone is a feldspar. Search in areas known for pegmatites, hydrothermal veins, or alpine fissures where feldspar minerals are common.
Observe Optical Effects
The key identifying feature is adularescence. Rotate potential specimens under a light source to look for the characteristic shimmering, bluish-white to silvery sheen.
Check for Cleavage
Feldspars have distinct cleavage planes. Look for two cleavage directions at or near 90 degrees.
Consider Associated Minerals
Moonstone can be found alongside other minerals typical of its formation environment, such as quartz, mica, and other feldspars.
Similar Rocks
Labradorite
(Ca,Na)(Al,Si)4O8
Also known as: Spectrolite
Opal
SiO2·nH2O
Also known as: Precious Opal
Chalcedony
SiO2
Also known as: Agate, Carnelian
Scientific Classification
- Mineral Class
- Silicate
- Group
- Feldspar Group
- Crystal System
- Monoclinic (Orthoclase/Adularia) or Triclinic (Plagioclase)
- Chemical Formula
- KAlSi3O8 (Orthoclase/Adularia) or (Na,Ca)(Al,Si)4O8 (Plagioclase)
- Composition
- Potassium aluminum silicate (Orthoclase/Adularia); Sodium calcium aluminum silicate (Plagioclase).
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