Not all metamorphic rocks have luster or shiny grains. “Non-foliated” rocks are often opaque and dull in color.
These aren’t like the layers of sedimentary rock, which have a texture to them, and appear as though the rock is made of stacked pieces.
If you can’t discern the smaller flecks, you can use a magnifier to see them in more detail. Metamorphic rocks like granite don’t have bands, but they do have remarkably concentrated amounts of crystals.
Slate is a particularly tricky rock, as it has many features in common with sedimentary rocks.
If the grains seem to be organized more heavily in one are than another, or seem to “flow” evenly around the rock, it is likely metamorphic.
The stripes or bands may be difficult to see, so be sure to look closely for any directions that crystals seem to be oriented in. Common foliated metamorphic rocks include slate, phyllite, and gneiss. Two common non-foliated metamorphic rocks are marble and quartzite.
The paleness of quartzite often appears almost yellow in color, due to chemical discoloration and impurities in the rock.
A gneiss rock will have a grey hue to it overall, with the dark and light strikes cutting through the gray coloration.
You only need to streak the rock against roughly 2 millimetres (0. 079 in) of glass.
The key to identifying marble is noticing the large crystals that seem randomly distributed, since it is a non-foliated form of metamorphic rock. Marble can vary widely in color, but the most common colors are white and grey.
Slate is typically gray, black, or green. The gray color is a distinctive shade, often called “slate. ” The layers in slate are not formed from sedimentation, but from the organization of crystal molecules into a straight line under immense pressure.
The color alone can’t be used to distinguish the two, but it can help improve your confidence in the analysis.
