Coal (most likely Bituminous or Anthracite)

Hardness: 1-2.5 (Bituminous), 2.5-2.75 (Anthracite) on Mohs scale; Color: Black, dark gray-black; Luster: Dull to sub-vitreous (bituminous), bright to vitreous (anthracite); Crystal structure: Amorphous to microcrystalline; Cleavage: None (fractures irregularly or conchoidally); Specific Gravity: 1.2-1.5 (bituminous), 1.3-1.8 (anthracite). The specimen appears dense and blocky with a somewhat dull to sub-vitreous luster, suggesting it could be bituminous coal, or possibly a lower grade of anthracite.

Coal forms from the accumulation and preservation of plant matter, often in swampy environments. This organic material is then subjected to heat and pressure over millions of years (a process called coalification). Lignite forms first, then sub-bituminous, followed by bituminous coal, and finally anthracite, which is the highest grade. The geological age of coal deposits varies, but significant deposits formed during the Carboniferous Period (approx. 359 to 299 million years ago), often referred to as the 'Coal Age'.

Primarily used as a major fuel source for electricity generation and industrial processes. Bituminous coal is also used in the production of coke for the steel industry. Anthracite is a cleaner-burning and higher energy content fuel, often used for domestic heating. Historically, it was crucial for powering steam engines and early industrialization.

Coal is the largest source of fuel for electricity worldwide. Different grades of coal (lignite, sub-bituminous, bituminous, anthracite) are classified based on their carbon content and energy value, which increases with the degree of coalification. The formation of coal traps significant amounts of carbon, and its combustion releases this carbon back into the atmosphere as CO2, contributing to global warming. Coal seams can sometimes contain fossilized plant remains.

In the field, coal is identified by its black color, relatively low density (it feels lighter than typical rocks of similar size), its often layered or banded appearance (especially bituminous coal), and its tendency to be somewhat brittle. It leaves a black streak. Anthracite will have a harder, more lustrous appearance and is less friable than lower grades. Common locations include regions with ancient swamp deposits, such as the Appalachian Basin in the USA, parts of Russia, China, India, and Australia. Collectors may seek specimens that show clear plant fossils or interesting fracture patterns. It can also be identified by its combustibility; a small piece will usually burn if ignited.