Final answer:
Anthracite is the metamorphosed form of coal, representing the highest grade due to higher carbon content and heating value. It is used in metallurgical refining and is different from other coal types, such as lignite and bituminous, which have lower energy content and can contribute to acid rain.
Step-by-step explanation:
The metamorphosed form of coal is anthracite. Coal is a fossil fuel composed mainly of carbon and hydrocarbons formed from the remains of ancient plant life that was subjected to high pressure and temperature over millions of years. Peat is the precursor to coal and through the coalification process, it transforms into lignite, then to bituminous coal, and finally to anthracite, which is the highest grade of coal due to its higher carbon content and heating value.
Anthracite coal is preferred for metallurgical refining because of its higher energy content and lower sulfur content compared to other forms like lignite and bituminous coal. Bituminous coal is the most abundant in the United States and contains a high amount of sulfur, often leading to acid rain when burned. In contrast, gneiss is a type of high-grade metamorphic rock that does not originate from coal, but rather from the intense metamorphism of other rock types such as schists or granites.