An alloy of copper (90+%), tin, and various other elements. There are many formulations of alloys. Inaru uses alloy C510, a phosphor bronze.
Phosphor bronze has a lovely rose gold color. It will darken with wear and exposure but can easily be restored to its rosy beauty by a quick dip in vinegar.
In this picture, the top chain has been allowed to darken. The middle chain was cleaned with vinegar prior to the photo. The bottom chain is a few years old and has been re-brightened with vinegar many times.
Bronze is a traditional maritime metal. It is harder than sterling silver but soft enough to be relatively easy to work. It does not have as strong a "new penny" smell as pure copper.