hard charcoal
Refers to a type of charcoal that is dense and burns slowly, often used for cooking or heating.
Hard charcoal is often used in yakitori restaurants because it burns for a long time.
Compound of 堅 (かた, 'hard') and 炭 (ずみ, 'charcoal'). The reading ずみ is a voiced form of すみ (炭) due to rendaku.