Gneiss is one of the planet’s most incredibly common and recognizable metamorphic rocks which means it’s pretty much everywhere. So, you’re probably wondering why I’m focusing on gneiss when it doesn’t have a lot of value and most people don’t collect random, everyday rocks.
If you’re a die-hard rockhound then you need to get good at identifying different varieties of rocks and you’ll want to know which types of rocks are prevalent when looking for minerals and gems. For example, if granite and schist need to be in the area where you’re digging for garnets then you’ll want to be able to quickly identify the rocks and minerals which should be present.
Back to Gneiss minerals, it comes in various varieties and sometimes contains attractive gemstone inclusions.
While Gneiss is a common rock, it can be challenging to identify, partly because it’s often confused with other tock types.
Gneiss Rock Type
Gneiss are foliated metamorphic rocks identified by their lenses and bands of varying mineral compositions. Some of these lenses or bands contain granular minerals bound together in an interlocking texture.
However, other bands contain elongated or platy minerals that display a preferred orientation that parallels the overall banding within the rocks. Combined with the texture, this banded appearance defines a gneiss instead of the stone’s composition.
Gneiss rocks are formed at high temperatures and pressures. When visually inspecting the rock you’ll notice a banded texture characterized by alternating darker and lighter colored bands, all without distinct cleavage.
Because Gneiss isn’t defined by its mineralogy, it comes in more than one color. The type of Gneiss and relative proportions of the minerals within drastically impact the stone’s colors.
Light-colored minerals in Gneiss are nearly always Feldspar and Quartz. Quartz is almost always an off-white color and can be translucent. It’s usually a significant contributing factor to the color of Gneisses’ light-colored bands.
While Feldspar comes in more varieties and colors than Quartz, the two primary groups of Feldspar are Plagioclase Feldspar and Alkali Feldspar. Plagioclase Feldspar is nearly always whitish in color, while Alkali is usually a salmon-pink to off-white color.
So, depending on the type and amount of Feldspar or Quartz present, the rocks can appear in different light-colored hues and shades.
Dark-colored bands in Gneiss are caused by mafic minerals like pyroxene, hornblende, or biotite. These minerals are generally dark gray or black, which is why there isn’t as much variation in the color of dark bands.
The ratio of dark to light bands significantly impacts Gneisses’ appearance. Some stones have considerably more light bands compared to dark or vice versa. However, the bands are usually relatively evenly distributed.
Gneiss Rock Uses
Because gneiss doesn’t typically split along planes of weakness like many metamorphic rocks, it’s used as crushed stone.
Some of the various uses include:
- Road Construction
- Building Site Preparations
There are some Gneiss varieties that are durable enough to be used as dimension stones that are sheared or sawn into slabs and blocks and used for a variety of paving, building, and curbing projects.
Some Gneiss specimens accept a bright polish attractive enough to use as architectural stones. Stunning floor tiles, stair treads, facing stones, countertops, window sills, and even cemetery monuments are often made using polished gneiss.
Types of Gneiss Rocks
Types of gneiss rocks contain a large portion of gneiss and other minerals or stones.
There are actually many types of Gneiss rocks, but here is the shortlist.
- Cordierite-Garnet Gneiss
- Gabbro Gneiss
- Hornblende-Plagioclase Gneiss
- Muscovite Gneiss
- Nepheline-Bearing-Syenite Gneiss
- Banded Gneiss
- Augen Gneiss
- Agmatic Gneiss
- Lineated Gneiss
- Hornblende Gneiss
- Glaucophane Gneiss
- Biotite Gneiss
- Granitic Gneiss
The list goes on with various combinations of different stones, with the primary specimen being gneiss.
Is Gneiss Foliated or Non-Foliated?
Gneiss is a foliated metamorphic rock, meaning it’s made of multiple thin layers of rock.
Gneiss Rock Texture
Gneiss rock texture is a type of metamorphic foliation describing parallel bands of alternating (light and dark) minerals with a poorly developed schistosity. The mineral grains in the stone are coarse and visible to our naked eye.
One aspect that’s often overlooked is Gneisses’ crystal size. For a rock to be considered a variety of Gneiss, we must be able to see the individual crystals without magnification (unless we have poor eyesight). Suppose we can’t see any crystals without a hand lens. In that case, we might be looking at phyllite, a precursor to schist in the metamorphic process.
A metamorphic rock that’s yet to develop distinct alternating bands cannot be called or considered Gneiss.