Final answer:
Low-med grade metamorphic rocks are characterized by a shiny, dark gray to black appearance, with features such as mixtures of quartz, feldspar, and fine to medium grain sizes. Gneiss is a high-grade metamorphic rock with banded appearance, unlike the smoother and flatter surfaced low-grade slate. Classification is aided by flowcharts like those from LibreTexts™, detailing properties such as color, texture, and mineral composition.
Step-by-step explanation:
The description provided points towards the characteristics of low-med metamorphic grade rocks, which often exhibit a shiny, dark gray to black appearance. These rocks can exhibit a mix of quartz, feldspar, and rock fragments, and may have a dark, oily smell.
Additionally, they can have very thin, irregular layers of mica, usually pale gray-green with a satin sheen that is not confined to individual flakes but rather to the rock as a whole. The grain size can be fine to medium, with uneven surfaces where grains are visible.
Gneiss is another type of metamorphic rock, but it is considered high-grade rather than low to medium grade. It is characterized by its banded appearance and segregation of light and dark minerals due to intense metamorphism. On the other hand, low-grade rocks like slate have smooth, flat surfaces, often with a slatey cleavage, dense texture, and a brittle nature that produces a clinking sound when struck.
According to LibreTexts™, one should determine whether metamorphic rocks are primarily high, medium, or low grade by examining characteristics such as texture, mineral composition, and the presence of certain minerals indicated in flowcharts for classification.
Finally, identifying the age of rocks, such as those in New Jersey of the Mount Laurel Granite or the Hornblende granite of the Byram Intrusive Suite, provides context for the geological history and conditions that led to the formation of the metamorphic rocks in question.