A good quality nickel wound strings
Long made of gut (of sheep, the term cat gut for violin strings is a translation error) and still used today in early music, cello strings are now made of either synthetic or metallic core. , yarn (covered) with a second material which makes the rope heavier while maintaining good flexibility
Stainless Steel
Nickel round-wound
Medium
High grade soft-tempered Swiss steel core
For 3/4 and 4/4 size cellos