Piano wire is wire that forms the piano strings. According to Wikipedia, "it is made from tempered high-carbon steel, also known as spring steel, which replaced iron as the material starting in 1834.
"Piano wire has a very high tensile strength to cope with the heavy demands placed upon piano strings; accordingly, piano wire is also used for a number of other purposes, including springs, surgical uses, and in special effects."
Or, in this case, for a pearl display.