Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Black Moonstone is a captivating variety of orthoclase feldspar, distinguished by its dark, often black, grey, or brownish-black body color, combined with the characteristic adularescence or 'schiller' effect. This optical phenomenon, a soft, glowing play of light resembling moonlight, appears to float across the surface of the stone as it is rotated. The dark coloration is due to minute inclusions within the feldspar matrix, which can also influence the color of the adularescence, sometimes giving it a silvery, bluish, or even brownish sheen. It is a potassium aluminum silicate and belongs to the feldspar group of minerals.
How to Identify
- Color
- Dark grey, black, brownish-black body color, often with a silvery, bluish, or brownish adularescence (schiller).
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Typically massive, sometimes in tabular or prismatic crystals. The adularescence is a key textural/optical feature.
- Crystal Form
- Monoclinic system, typically forming tabular or prismatic crystals, though often found in massive or anhedral forms in Black Moonstone varieties. Crystals can be twinned (e.g., Carlsbad, Baveno, Manebach twins).
- Cleavage
- Perfect cleavage in two directions, nearly at right angles (90 degrees).
- Geological Environment
- Found in igneous rocks (e.g., granites, syenites, pegmatites) and some metamorphic rocks (e.g., gneisses). The specific conditions for the formation of the dark inclusions are varied but generally involve the presence of iron-rich fluids or minerals during crystallization or subsequent alteration.
Key Facts
- Hardness: 6 - 6.5 (Mohs scale)
- Specific Gravity: 2.56 - 2.59
- Crystal System: Monoclinic
- Color: Dark grey, black, brownish-black with adularescence
- Luster: Vitreous to pearly
- Transparency: Translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions, nearly at 90 degrees ({001} and {010})
- Composition: Potassium aluminum silicate (KAlSi3O8) with fine mineral inclusions
Quick Check
- Color: Dark grey to black with adularescence
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Typically massive, anhedral grains, or tabular to prismatic crystals; often twinned.
- Cleavage Type: Perfect in two directions, nearly at right angles (90 degrees).
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces.
Formation
Black Moonstone is a variety of orthoclase feldspar, a tectosilicate mineral. Its dark color is attributed to fine inclusions of other minerals, often hematite, ilmenite, or other iron-rich minerals, which are dispersed within the feldspar structure. The characteristic adularescence (schiller or 'moonstone effect') is caused by the diffraction of light off microscopic lamellar intergrowths of two different feldspar species (orthoclase and albite) with slightly different refractive indices. The black variety specifically owes its dark body color to the aforementioned inclusions.
Usage
Primarily used as a gemstone in jewelry due to its unique adularescence and dark body color. It is also collected by mineral enthusiasts. It does not have significant industrial applications distinct from other feldspars.
Age Distribution
Found in various geological ages, typically associated with igneous and metamorphic rock formations.
Where to Find
India
Significant source of various moonstone varieties, including those with darker body colors.
Sri Lanka (Ceylon)
Historically a major source of high-quality moonstone, some darker varieties may be found.
Madagascar
Known for producing various feldspar minerals, including moonstone with diverse appearances.
Myanmar (Burma)
Another historical source of moonstone.
Tanzania
Emerging source for various gemstones, including some darker feldspar varieties.
Finding Tips
Look for Adularescence
The most defining characteristic is the adularescence. Rotate the specimen under a light source to observe the characteristic floating sheen. Without this, it's likely not moonstone.
Examine Body Color
Confirm the dark body color (black, dark grey, brownish-black) to distinguish it from other moonstone varieties.
Check for Cleavage
Feldspars exhibit perfect cleavage in two directions at approximately 90 degrees. This can be observed on broken surfaces.
Consider Hardness
With a Mohs hardness of 6-6.5, it can be scratched by quartz but will scratch glass. This helps differentiate it from softer or harder black minerals.
Geological Context
Search in areas known for pegmatites, granites, or certain metamorphic rocks, as these are common host rocks for feldspars.
Similar Rocks
Labradorite
Plagioclase Feldspar (Na,Ca)(Al,Si)AlSi2O8
Also known as: Spectrolite (a high-quality variety)
Rainbow Moonstone
Plagioclase Feldspar (Na,Ca)(Al,Si)AlSi2O8
Also known as: White Labradorite
Obsidian
Amorphous volcanic glass
Also known as: Volcanic Glass
Black Tourmaline
Na(Fe2+3)Al6(BO3)3Si6O18(OH)4
Also known as: Schorl
Scientific Classification
- Mineral Class
- Tectosilicate
- Group
- Feldspar Group (Potassium Feldspar Subgroup)
- Crystal System
- Monoclinic
- Chemical Formula
- KAlSi3O8 (Orthoclase end-member)
- Composition
- Potassium aluminum silicate, with trace elements and fine mineral inclusions (e.g., hematite, ilmenite) responsible for the dark color.
Explore Black Moonstone
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.