metamorphic
Gneiss
Metamorphic Gneiss
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating gray, white, and pinkish layers; Luster: dull to pearly; Crystal structure: crystalline but with distinct foliation; Cleavage: poor; Specific gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- dull to pearly
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Physical properties
Hardness: 6-7 on Mohs scale; Color: typically banded with alternating gray, white, and pinkish layers; Luster: dull to pearly; Crystal structure: crystalline but with distinct foliation; Cleavage: poor; Specific gravity: 2.6-2.9.
Formation & geological history
Formed by high-grade regional metamorphism of pre-existing igneous or sedimentary rocks (like granite or shale). Subjected to intense heat and pressure, causing minerals to segregate into bands. Can be billions of years old, often forming the cores of mountain ranges.
Uses & applications
Used in construction for road aggregates, building stone, and dimension stone for countertops or monuments. It is durable and aesthetically pleasing when polished.
Geological facts
Gneiss is one of the oldest types of rock found on Earth; the Acasta Gneiss in Canada is dated at approximately 4.03 billion years old. The term 'gneiss' comes from a Middle High German verb meaning 'to sparkle'.
Field identification & locations
Identified in the field by its distinct banding (gneissic foliation) and visible crystals of quartz, feldspar, and mica. Commonly found in shield areas of continents and deeply eroded mountain belts. Collectors look for sharp, high-contrast mineral banding.
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