Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Sunstone is a variety of plagioclase feldspar, specifically oligoclase or labradorite, that exhibits a distinctive optical effect known as aventurescence or schiller. This phenomenon is characterized by a glittering or spangled appearance caused by the reflection of light from minute, platy inclusions within the mineral. These inclusions are typically hematite or goethite, but in the case of Oregon Sunstone, they are often microscopic platelets of native copper. The color of sunstone can range from colorless or yellow to orange, pink, red, and even green or blue, depending on the composition and size of the inclusions and the body color of the feldspar. The intensity of the aventurescence varies with the density and orientation of these inclusions.
How to Identify
- Color
- Typically yellow, orange, pink, red, or reddish-brown, often with a metallic glitter. Can also be colorless, green, or blue.
- Luster
- Vitreous to sub-vitreous, with a distinct metallic glitter (aventurescence) from inclusions.
- Texture
- Crystalline, often massive or granular. Individual crystals can be tabular or prismatic.
- Crystal Form
- Typically anhedral to subhedral grains within igneous or metamorphic rocks. When well-formed, it exhibits triclinic crystal habits, often tabular or prismatic.
- Cleavage
- Perfect in two directions, nearly at 90 degrees (86 degrees and 94 degrees), characteristic of plagioclase feldspars.
- Geological Environment
- Found in mafic igneous rocks (e.g., basalts, gabbros), felsic igneous rocks (e.g., granites, pegmatites), and some metamorphic rocks. The most famous occurrences are in basaltic lava flows where it forms phenocrysts.
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, pink, red, reddish-brown, green, blue, often with metallic glitter.
- Luster: Vitreous to sub-vitreous, with aventurescence (schiller effect).
- Transparency: Transparent to translucent
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions (pinacoidal), nearly at 90 degrees (86° and 94°).
- Composition: Sodium calcium aluminum silicate (Na,Ca)(Al,Si)AlSi2O8)
Quick Check
- Color: Yellow, orange, pink, red, reddish-brown, sometimes green or blue, with metallic glitter.
- Luster: Vitreous to sub-vitreous, with aventurescence.
- Streak: White
Physical Characteristics
- Crystal Habit: Typically anhedral to subhedral grains, massive, granular. Can form tabular or prismatic crystals.
- Cleavage Type: Perfect on {001} and {010}, nearly at right angles.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to sub-vitreous, with aventurescence.
Formation
Sunstone forms in igneous rocks, particularly in mafic lavas (basalts) and intrusive rocks, as well as in some metamorphic rocks. The characteristic aventurescence (schiller effect) is due to inclusions of hematite or goethite platelets, typically oriented along crystallographic planes. In Oregon Sunstone, the inclusions are often copper platelets. These inclusions are exsolved from the host plagioclase during cooling or later metamorphic processes.
Usage
Primarily used as a gemstone for jewelry due to its unique optical phenomenon (aventurescence). High-quality specimens are faceted, while lower-grade material is often cut into cabochons or beads. It is also collected by mineral enthusiasts.
Age Distribution
Sunstone formation is not tied to a specific geological age but rather to specific geological processes. It can be found in rocks ranging from Precambrian to Cenozoic.
Where to Find
Oregon, USA
Famous for 'Oregon Sunstone,' which contains native copper inclusions, producing unique red, green, and bi-color effects. Found in basaltic lava flows in Harney and Lake Counties.
Norway
Historically significant source, particularly for oligoclase sunstone with hematite inclusions, found in pegmatites and metamorphic rocks.
India
Produces sunstone with hematite inclusions, often found in metamorphic terrains.
Tanzania
Known for sunstone with strong aventurescence, often with reddish-orange hues.
Canada
Occurrences in various provinces, often associated with anorthosite complexes.
Russia
Found in certain igneous and metamorphic rock formations.
Madagascar
Produces sunstone with good clarity and aventurescence.
Finding Tips
Look for Igneous Rocks
Sunstone is most commonly found in mafic igneous rocks like basalt flows or in some granitic pegmatites. Search in areas known for these rock types.
Identify the Schiller Effect
The most distinctive feature is the aventurescence. Rotate the specimen under a light source to observe the metallic glitter from the inclusions. This is key to distinguishing it from other feldspars.
Check for Cleavage
Like all feldspars, sunstone exhibits two perfect cleavages at nearly 90 degrees. This can be observed on broken surfaces.
Examine Inclusions
Under magnification, the platy inclusions (hematite, goethite, or copper) responsible for the schiller effect may be visible. Their orientation often aligns with crystallographic planes.
Consider Geographic Location
Focus your search on known sunstone localities, such as the basalt fields of Oregon or pegmatite districts in Norway or India.
Similar Rocks
Aventurine
Aventurine Quartz
Also known as: Green Aventurine, Red Aventurine
Goldstone
Synthetic Glass
Also known as: Monk's Gold
Moonstone
Orthoclase or Albite-Orthoclase Intergrowth
Also known as: Adularia, Peristerite
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 typically oligoclase (An10-An30) or labradorite (An50-An70) with specific inclusions.
Explore Sunstone
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.