This is a skin for the Rock item. You will be able to apply this skin when you craft the item in game.
The Rock is practically the mascot of Rust. As both a weapon and tool, every player starting with one must use it to initially collect resources and protect themselves from the environment before upgrading to superior utensils. The Rock, if broken, is very cheap to craft, costing only a few stones.
It is unarguably the least efficient tool and weapon in the game. With its low gather rates and slow speed, most players will desire to immediately upgrade to at least stone tools once they have acquired enough resources.