Gneiss containing Garnet and Biotite
Metamorphic Rock var. Gneiss with Almandine Garnet and Biotite · Mineral Specimen / Metamorphic Rock
Crystal System
Non-crystalline (Rock fabric is Foliated); individual minerals are Isometric (Garnet) and Monoclinic (Biotite)
Mineral Group
Silicates (specifically Nesosilicates for Garnet and Phyllosilicates for Biotite)
Chemical Formula
Complex Silicate mixture: X3Y2(SiO4)3 (Garnet) + K(Mg,Fe)3AlSi3O10(OH,F)2 (Biotite)
Formation Process
Metamorphic: formed under high temperature and pressure through the regional metamorphism of shale or granitic precursors
About This Crystal
A medium-to-coarse-grained foliated rock showing distinct banding or layering. Displays an earthy to sub-vitreous luster with small, dark granular inclusions of biotite and reddish-brown garnet specks on a grey-green matrix.
Physical Characteristics
Color: Grey-green, tan, and reddish-brown; Streak: White; Fracture: Uneven; Specific Gravity: 2.6-3.0; Tenacity: Brittle; Fluorescence: Generally inert
Optical Properties
Generally opaque as a hand specimen; individual garnet crystals may show high relief and single refraction (isotropic) under thin section examination.
Hardness & Durability
Origin Region
Global; common in high-grade metamorphic terranes like the Adirondacks (USA), Scandinavia, and the Himalayas
Hardness & Durability
Variable by mineral: Garnet (6.5-7.5), Matrix (5-6). Overall high durability but prone to splitting along foliation planes.
Care & Maintenance
Clean with warm soapy water and a soft brush; avoid harsh chemicals that could dissolve softer mineral components; store in a dry environment.
Rarity & Value
Common; primary value is as a geological specimen or decorative landscaping stone. Minimal gemstone value for this specific rough matrix.
Special Characteristics
Porphyroblastic texture (large crystals in a finer matrix) and evident foliation/gneissic banding resulting from tectonic pressure.
Lore & History
Gneiss is one of the oldest rock types on Earth, with some specimens dating back 4 billion years. Historically used for building stone and monuments due to its strength.