Gneiss (Polymineralic Metamorphic Rock)
Gneiss (Intermediate to High-Grade Metamorphic Rock) · Mineral Specimen / Common Decorative Rock
Crystal System
Variable; contains multiple crystal systems (Trigonal, Triclinic, Monoclinic) depending on constituent minerals. Typical habit is foliated/gneissic banding.
Mineral Group
Silicates (Non-mineralic; lithic specimen composed primarily of Tectosilicates and Phyllosilicates)
Chemical Formula
Variable; typically SiO2 (Quartz), KAlSi3O8 (Feldspar), and K(Mg,Fe)3AlSi3O10(OH)2 (Biotite)
Formation Process
Metamorphic; forms through regional metamorphism of igneous or sedimentary protoliths under high temperature and high pressure conditions deep within the crust.
About This Crystal
Opaque rock with a coarse-grained texture. It displays characteristic gneissic banding—alternating layers of light-colored minerals (quartz/feldspar) and dark minerals (biotite/amphibole). Luster is generally dull to earthy, with occasional vitreous glints from silicate faces.
Physical Characteristics
Color: Mottled brown, grey, and tan; Streak: White (from silicates); Cleavage: None (rock mass), though constituent mica has perfect basal cleavage; Fracture: Uneven/Rough; Specific Gravity: 2.6–3.0; Fluorescence: Generally none.
Optical Properties
Opaque in hand specimen; individual constituent minerals like quartz are singly refractive or doubly refractive depending on the specific mineral grain.
Hardness & Durability
Origin Region
Found globally in continental shields like the Canadian Shield or Scandinavian Highlands; typically forms in the roots of mountain ranges.
Hardness & Durability
Mohs Hardness: 6–7 (dominated by quartz/feldspar); High toughness due to interlocking crystalline structure; Stable for most decorative uses.
Care & Maintenance
Wash with warm soapy water; very durable and resistant to most household chemicals, though dark mica layers can be scratched by harder materials. Avoid prolonged exposure to strong acids.
Rarity & Value
Very common; low monetary value as a specimen. Used primarily as a construction material (crushed stone) or for landscaping.
Special Characteristics
Displays foliation known as 'gneissic banding,' which results from the segregation of minerals during high-grade metamorphism.
Lore & History
Used historically by ancient civilizations as building stone and for statuary due to its durability. In metaphysical traditions, it is often associated with finding harmony within change and grounding one's energy to Earth's ancient history.