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SoyBooru
Obsidian and Coal differ fundamentally in origin and composition. Obsidian is a naturally occurring volcanic glass formed when lava cools so rapidly that crystals don't have time to form; it is primarily composed of silica (SiO2). Coal, on the other hand, is a sedimentary rock formed from ancient plant material that was buried, compressed, and heated over millions of years. Chemically, coal is mostly carbon along with hydrogen, sulfur, oxygen, and nitrogen compounds, and its exact makeup varies depending on its type (peat, lignite, bituminous, or anthracite).
In terms of physical properties, obsidian is much harder and denser than most coal types. Obsidian typically has a density of about 2.3-2.6 g/cm3 and ranks around 5-5.5 on the Mohs hardness scale. Coal is generally less dense, ranging from about 1.1-1.5 g/cm3 depending on grade, and is much softer (around 1-3 on the Mohs scale). Obsidian has a glassy texture and breaks with sharp, conchoidal (curved) fractures, which is why it was historically used for cutting tools. Coal is dull to shiny (depending on type), brittle, and tends to leave a black streak.
Their thermal properties are also very different. Obsidian does not burn because it is not a fuel; instead, it melts at very high temperatures (roughly 700-1,200degC depending on composition). Coal, by contrast, is highly combustible and burns at temperatures typically between about 1,000-2,000degC in air, releasing significant heat energy, which is why it has been widely used as a fossil fuel for electricity and steel production. In short, obsidian is a hard volcanic glass used mainly for tools and decorative purposes, while coal is a soft, carbon-rich rock primarily valued as a source of energy.