metamorphic
Gneiss
High-grade foliated metamorphic rock
Hardness: 6-7 (Mohs scale); Color: Alternating dark and light bands of gray, pink, tan, or black; Luster: Vitreous to dull; Crystal structure: Coarse-grained crystalline with distinct foliation; Cleavage: Generally poor, breaks along foliated planes; Specific gravity: 2.6-2.9
- Hardness
- 6-7 (Mohs scale)
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Alternating dark and light bands of gray, pink, tan, or black; Luster: Vitreous to dull; Crystal structure: Coarse-grained crystalline with distinct foliation; Cleavage: Generally poor, breaks along foliated planes; Specific gravity: 2.6-2.9
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale) under intense heat (600-700°C) and pressure. This specimen shows a specific banded texture called 'Gneissic Banding'.
Uses & applications
Extensively used in construction as crushed stone for road bases or as ornamental building stone (countertops, tiles, and monuments) due to its durability and aesthetic patterns. Often sold commercially as 'Granite'.
Geological facts
Gneiss includes some of the oldest known rocks on Earth, such as the Acasta Gneiss in Canada, which is roughly 4.03 billion years old. This specific specimen displays 'Augen' (eye-like) structures where large crystals of feldspar have been stretched.
Field identification & locations
Identified in the field by its distinct banded appearance ('Gneissic Banding') and large visible crystals. Commonly found in the crystalline cores of mountain ranges and stable continental shields. Collectors look for unique banding patterns or 'Augen' textures.
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