Gneiss

Hardness: 6-7 on Mohs scale (for quartz and feldspar components). Color: Predominantly black and white, often in distinct bands or swirling patterns. Luster: Vitreous to dull, depending on mineralogy. Crystal structure: Crystalline, with interlocking grains, often showing foliation (banded appearance) or lineation. Cleavage: Not applicable for the rock as a whole, but individual minerals like feldspar may show cleavage. Specific Gravity: Typically 2.6-3.0.

Gneiss forms from the metamorphism of pre-existing igneous or sedimentary rocks under high temperature and pressure conditions. This process, known as regional metamorphism, typically occurs during mountain-building events (orogenies) or deep burial within the Earth's crust. The protolith (original rock) can be granite (forming orthogneiss) or shale/sandstone (forming paragneiss). Geological age: Gneiss can be found from very ancient Precambrian ages (billions of years old) to more recent metamorphic events.

Industry: Dimension stone for building facings, tiles, and monuments due to its durability and attractive patterns. Construction: Crushed for road aggregate and railway ballast. Jewelry: Not typically used as a gemstone, but attractive pieces might be polished as ornamental stones. Collecting: Specimens with distinctive banding or augen structures are popular among collectors.

Gneiss is one of the most common rocks in continental crust. It often forms the basement rock of mountain ranges and ancient shields. The 'eye' structures sometimes seen in gneiss are called augen (German for 'eyes') and are large, typically feldspar, porphyroclasts embedded in a finer-grained matrix. Some gneisses, particularly those with a high proportion of quartz and feldspar, can resemble granite from a distance, but the banding distinguishes them.

How to identify in the field: Gneiss is best identified by its distinctive banding (foliation) of light and dark minerals, which are typically quartz, feldspar (light bands), and mica, hornblende, or other dark minerals (dark bands). The bands are often wavy, contorted, or discontinuous. Common locations: Found globally in areas that have experienced significant tectonic activity and deep burial, such as ancient shield areas (e.g., Canadian Shield, Baltic Shield), and the cores of mountain ranges (e.g., Appalachians, Himalayas). Tips for collectors: Look for samples with well-defined banding and interesting mineral compositions. As it is quite hard, it can be challenging to break without proper tools.