metamorphic
Gneiss (specifically Pink Gneiss)
Gneiss (high-grade regional metamorphic rock)
Hardness: 6-7 on Mohs scale. Color: Distinct banding (gneissose banding) of pink (orthoclase feldspar), white (quartz), and black (biotite/hornblende). Luster: Dull to vitreous. Structure: Coarse-grained, foliated with alternating light and dark mineral layers.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Distinct banding (gneissose banding) of pink (orthoclase feldspar), white (quartz), and black (biotite/hornblende). Luster: Dull to vitreous. Structure: Coarse-grained, foliated with alternating light and dark mineral layers.
Formation & geological history
Formed through high-grade regional metamorphism where pre-existing igneous rocks (like granite) or sedimentary rocks are subjected to extreme heat and pressure. This specimen likely dates back to the Precambrian or Paleozoic eras depending on the local shield geology.
Uses & applications
Used primarily as crushed stone for road construction and concrete aggregate. High-quality slabs are used for countertops, flooring, and ornamental building stone (often sold under the trade name 'Granite').
Geological facts
Gneiss forms at temperatures above 600°C and represents one of the highest degrees of metamorphism before a rock begins to melt into magma. Some of the oldest rocks on Earth, such as the Acasta Gneiss in Canada (4 billion years old), are types of gneiss.
Field identification & locations
Identify in the field by looking for 'segregated banding'—distinct parallel stripes of different minerals that are often contorted or wavy. Found globally in mountainous regions and continental shields. Collectors look for unique 'eye' structures known as augen.
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