Gneiss
Metamorphic Rock (Gneissic texture) · Mineral Specimen / Decorative Stone / Industrial Mineral
Crystal System
Non-applicable (Polycrystalline rock); individual minerals within may be Trigonal, Monoclinic, or Triclinic. Habit is massive with a gneissic foliation/banding.
Mineral Group
Silicates (primarily Tectosilicates and Phyllosilicates)
Chemical Formula
Variable; typically SiO2 (Quartz), KAlSi3O8 (Orthoclase), and NaAlSi3O8 to CaAl2Si2O8 (Plagioclase) with (Mg, Fe)7Si8O22(OH)2 (Amphibole) or K(Mg,Fe)3(AlSi3O10)(OH)2 (Biotite Mica)
Formation Process
Metamorphic; specifically high-grade regional metamorphism of igneous (orthogneiss) or sedimentary (paragneiss) protoliths under high temperature and high pressure.
About This Crystal
A coarse-grained, foliated metamorphic rock characterized by distinct banding ('gneissic banding') of light-colored minerals (quartz, feldspar) and dark-colored minerals (biotite, hornblende). It is opaque with a dull to vitreous luster on fresh surfaces and a rough, granular surface texture.
Physical Characteristics
Color: Grey, white, pink, and black banding; Streak: White (from silicate content); Cleavage: None (rock mass), though mica components have perfect basal cleavage; Fracture: Irregular to splintery; Specific Gravity: 2.6 - 2.9; Non-fluorescent.
Optical Properties
Opaque in bulk; individual component minerals like quartz are refractive, but the rock itself does not exhibit collective optical phenomena like chatoyancy or play of color.
Hardness & Durability
Origin Region
Globally distributed; prevalent in eroded shield areas like the Canadian Shield, Scandinavia, and the Himalayas. Forms in deep crustal environments.
Hardness & Durability
Mohs hardness 6 to 7 (governed by quartz/feldspar content); high toughness; very stable against chemicals and sunlight; excellent wearability as a building material.
Care & Maintenance
Extremely durable; can be cleaned with warm soapy water and brushes. Resistant to most household chemicals, though acidic cleaners should be avoided if carbonate minerals are present as trace inclusions.
Rarity & Value
Very Common; low monetary value as a specimen, primarily valued as a construction aggregate, dimension stone for countertops, or landscaping material.
Special Characteristics
Gneissic banding (foliation) where minerals have segregated into layers due to tectonic pressure; may contain 'augen' (large eye-shaped mineral grains/phenocrysts).
Lore & History
Historically used as one of the oldest building materials in human history due to its strength. Metaphysically, it is traditionally associated with inner strength, grounding, and endurance through difficult transitions (symbolizing its metamorphic origin).