metamorphic
Diorite Gneiss
Foliated Diorite Gneiss
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper appearance with salt-colored plagioclase feldspar and pepper-colored hornblende or biotite. Luster: Dull to vitreous. Structure: Foliated with distinct banding or streaks. Cleavage: Poorly visible in massive hand samples.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper appearance with salt-colored plagioclase feldspar and pepper-colored hornblende or biotite. Luster: Dull to vitreous. Structure: Foliated with distinct banding or streaks. Cleavage: Poorly visible in massive hand samples.
Formation & geological history
Formed through the regional metamorphism of intermediate igneous rocks such as diorite. Subjected to high temperatures and pressures deep within the Earth's crust, causing the minerals to align and recrystallize into bands or streaks.
Uses & applications
Used primarily as crushed stone for road construction and aggregate. High-quality specimens with attractive banding may be used as dimension stone for countertops, flooring, and cemetery markers.
Geological facts
Gneiss is one of the oldest types of rock found on Earth, with some specimens dating back over 4 billion years. The word 'gneiss' comes from an Old High German verb meaning 'to spark' or 'to glisten.'
Field identification & locations
Identify by looking for the 'salt and pepper' mineralogy combined with distinct foliation (layering or streaks). Common in mountain belts and Precambrian shield areas. Collectors should look for sharp color contrast and well-defined banding.
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