Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Sunstone is a variety of plagioclase feldspar, specifically oligoclase, characterized by its distinctive aventurescence, a glittering or spangled effect caused by reflections from small, platy inclusions, usually hematite or goethite. These inclusions are typically oriented parallel to one crystallographic plane, producing a directional shimmer. The color of sunstone can range from colorless to yellow, orange, red, or brown, often with a metallic sheen. The intensity and color of the aventurescence vary depending on the size, density, and composition of the inclusions.
How to Identify
- Color
- Typically yellow, orange, red, or brown, often with a metallic or glittery sheen due to aventurescence. Can also be colorless.
- Luster
- Vitreous to sub-vitreous on cleavage surfaces, with a distinctive metallic or spangled aventurescent effect.
- Texture
- Crystalline, often granular in massive forms. The aventurescence is the key textural feature.
- Crystal Form
- Typically occurs as anhedral to subhedral grains within igneous or metamorphic rocks. Euhedral crystals are less common but can be tabular or prismatic. Cleavage fragments are common.
- Cleavage
- Perfect in two directions, intersecting at approximately 90 degrees (specifically, 86° and 94°), characteristic of plagioclase feldspar.
- Geological Environment
- Found in mafic igneous rocks (basalts, andesites, gabbros), some felsic igneous rocks (granites, granodiorites), and occasionally in metamorphic rocks. Also found in placer deposits where it has weathered out of its host rock.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale
- Specific Gravity: 2.62 to 2.67 g/cm³
- Crystal System: Triclinic
- Color: Colorless, yellow, orange, red, brown, often with a metallic glitter (aventurescence).
- Luster: Vitreous to sub-vitreous, with aventurescence.
- Transparency: Transparent to translucent.
- Fracture: Uneven to conchoidal.
- Cleavage: Perfect in two directions (pinacoidal), intersecting at approximately 86° and 94°.
- Composition: Sodium calcium aluminum silicate (Na,Ca)(Al,Si)AlSi2O8, specifically the oligoclase variety with 10-30% anorthite (CaAl2Si2O8) and 70-90% albite (NaAlSi3O8) end-members, plus inclusions of hematite or goethite.
Quick Check
- Color: Yellow, orange, red, brown, or colorless with a metallic glitter (aventurescence).
- Luster: Vitreous to sub-vitreous, with a distinct spangled or metallic sheen.
- Streak: White
Physical Characteristics
- Crystal Habit: Typically anhedral to subhedral grains; less commonly tabular or prismatic crystals. Massive forms are common.
- Cleavage Type: Perfect in two directions, {001} and {010}, intersecting at nearly 90 degrees (86° and 94°).
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to sub-vitreous, with aventurescence (schiller effect).
Formation
Sunstone forms primarily in igneous rocks, particularly in mafic lavas (e.g., basalt, andesite) and some felsic intrusive rocks (e.g., granite, granodiorite), as well as in metamorphic rocks and placers. The characteristic aventurescence (schiller effect) is due to inclusions of hematite or goethite, which are typically exsolved during cooling or later hydrothermal alteration.
Usage
Primarily used as a gemstone for jewelry due to its attractive aventurescence. It is cut into cabochons, beads, and faceted stones. Historically, it may have been used for ornamental purposes. It has no significant industrial uses beyond its role as a gemstone.
Age Distribution
Found in rocks of various ages, from Precambrian to Cenozoic, depending on the geological setting.
Where to Find
Oregon, USA
Known for its copper-bearing sunstone, which exhibits a unique range of colors including green, red, and schiller (aventurescence) in a single stone. Found in basalt flows.
Norway
Historically significant source, particularly for red and orange sunstone with hematite inclusions. Found in pegmatites and metamorphic rocks.
India
Produces sunstone with hematite inclusions, often from pegmatitic and metamorphic environments.
Tanzania
Known for producing sunstone with strong aventurescence.
Canada (Quebec)
Occurrences of sunstone have been noted in certain igneous and metamorphic terrains.
Russia
Found in various regions, often associated with igneous complexes.
Finding Tips
Look for Igneous Rocks
Sunstone is most commonly found in mafic igneous rocks like basalt and andesite, or in some granitic pegmatites. Search in areas with exposed volcanic or intrusive igneous formations.
Identify Placer Deposits
Due to its relative hardness and resistance to weathering, sunstone can accumulate in alluvial (river) or glacial deposits. Look in stream beds, gravel pits, and beach sands downstream from known occurrences.
Observe for Schiller Effect
The most distinctive feature is the aventurescence. Rotate potential specimens under a light source to check for the characteristic metallic glitter or spangled effect. This is often best seen in direct sunlight.
Check for Cleavage
Like all feldspars, sunstone exhibits two distinct cleavage planes at nearly 90 degrees. This can help differentiate it from quartz, which lacks cleavage.
Consider Associated Minerals
In igneous environments, sunstone may be found alongside other feldspars, pyroxenes, amphiboles, and quartz. In placer deposits, it might be mixed with other durable minerals.
Similar Rocks
Aventurine Quartz
Quartz (SiO2)
Also known as: Green Aventurine, Blue Aventurine
Goldstone
Synthetic glass with copper inclusions
Also known as: Monk's Gold, Aventurine Glass
Labradorite
Plagioclase Feldspar (Labradorite variety)
Also known as: Spectrolite
Scientific Classification
- Mineral Class
- Silicates
- Group
- Feldspar Group (Plagioclase Series)
- Crystal System
- Triclinic
- Chemical Formula
- (Na,Ca)(Al,Si)AlSi2O8
- Composition
- A solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Sunstone is specifically the oligoclase variety, meaning it contains 10-30% anorthite and 70-90% albite. The aventurescence is caused by microscopic inclusions of hematite (Fe2O3) or goethite (FeO(OH)).
Explore Sunstone
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.