Thomastik Spirocore Cello C: 

Spirocore strings have a flexible multiwire spiral rope core. They are highly responsive and long lasting.. They are especially popular with cellists who need a great deal of brilliance. The cello G tungsten and cello C tungsten are high-tension strings with a big sound. The cello silver G and cello silver C are a bit more mellow with less of an edge to their sound.