Final answer:
Anthracite coal is referred to as 'black diamond' due to its purity, high carbon content, and superior energy value among coal types, but it is now rarely found due to extensive consumption.
Step-by-step explanation:
The variety of coal that is sometimes referred to as 'black diamond' is anthracite coal. This is because anthracite is the hardest and most pure form of coal. It has the highest carbon content and the least impurities, giving it the highest heat value amongst the coal types. Out of the four main classes of coal, which include anthracite, bituminous, subbituminous, and lignite, anthracite has the greatest energy content and value, however, it has been largely depleted and is now a rare find.
When compared to other coals, anthracite has the highest carbon content, typically between 86% and 98%, and as a result, it burns cleaner than other forms of coal, producing fewer pollutants. Unfortunately, due to its high quality in terms of energy content, anthracite has been extensively harvested and is not as readily available as it once was. It is still sought after for applications such as home heating and metallurgical refining due to its high energy content.