Complex sentence: Firesteel is a piece of metal, often but not always high-carbon steel, used for striking a spark, sometimes kept in a tinderbox together with a striker or scrapers (sometimes made offlint) and tinder.
Split into simpler sentences: Firesteel is a piece of metal, often but not always high-carbon steel, used for striking a spark. Sometimes firesteel is kept in a tinderbox together with a striker or scrapers and tinder.