Coal (most likely Bituminous or Anthracite)

Hardness: 1-2.5 (Mohs scale); Color: Black to dark brown; Luster: Dull, sub-metallic, or vitreous (especially anthracite); Crystal structure: Amorphous, no true crystal structure; Cleavage: None, generally fracture conchoidally or irregularly; Specific Gravity: 1.1-1.6

Coal forms from the accumulation and burial of plant matter in oxygen-deficient environments, such as swamps and peat bogs. Over millions of years, heat and pressure transform the organic material through several stages (peat, lignite, sub-bituminous, bituminous, and anthracite). The geological age typically ranges from the Carboniferous Period (approx. 360-300 million years ago) to the Tertiary Period (65-2.6 million years ago), with significant deposits from the Carboniferous.

Predominantly used as a fossil fuel for electricity generation, industrial heat, and steel production (coking coal). Historically, it was a primary source of heat for homes and locomotives. Also used in the production of activated carbon and some chemical products.

Coal is the largest source of fuel for electricity generation worldwide. The amount of carbon in coal increases from lignite to anthracite, which also increases its energy content. The Carboniferous Period is often referred to as the 'Coal Age' due to the vast coal deposits formed during this time.

In the field, coal is identified by its black color, relatively low density (can feel light for its size), ease of breaking, and its characteristic dull or shiny (for anthracite) luster. It often leaves a black streak. It is commonly found in sedimentary basins around the world, particularly in regions that once had extensive ancient swamp forests. Major coal-producing regions include China, USA, India, Australia, and Russia. For collectors, higher grades like anthracite are more valued for their appearance, but all forms are common.