Calcium carbide  is a stony hard, very reactive substance produced from lime and coal.

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Calcium carbide reacts very active with water to form acetylene gas.
*  Calcium carbide was raw material for chemists when petroleum was not yet abundant.
*  Calcium carbide was used for welding and cutting of steel.
*  Carbide lights  ("lime lights") were needed for illumination
In case of a petroleum shortage calcium carbide might be important again.