metamorphic
Gneiss (specifically a weathered Amphibolite or Garnet Gneiss)
Gneiss (complex silicate composition)
Hardness: 6-7 (Mohs scale); Color: Black, dark gray, and tan/orange-brown; Luster: Dull to vitreous; Structure: Foliated with distinct banding or streaks; Cleavage: Generally poor, breaks along foliation planes.
- Hardness
- 6-7 (Mohs scale)
- Color
- Black, dark gray, and tan/orange-brown
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Black, dark gray, and tan/orange-brown; Luster: Dull to vitreous; Structure: Foliated with distinct banding or streaks; Cleavage: Generally poor, breaks along foliation planes.
Formation & geological history
Formed through high-grade regional metamorphism of igneous or sedimentary rocks at high temperatures and pressures. These rocks are often billions of years old, originating from deeply buried Precambrian shield regions.
Uses & applications
Used as crushed stone for road construction, concrete aggregate, and decorative landscaping. High-quality slabs are used for countertops and architectural facing.
Geological facts
Gneiss is some of the oldest rock on Earth, with the Acasta Gneiss in Canada dated to approximately 4 billion years old. Because of its extreme depth of formation, it reveals secrets of the Earth's early tectonic history.
Field identification & locations
Identify in the field by checking for 'foliation'—parallel bands of different minerals. This weathered specimen shows differential erosion where harder minerals protrude from the softer matrix. Common in glacial till or riverbeds near mountain ranges.
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