The Rock was the first harvesting tool you could use in Legacy. Unlike other tools, it was unbreakable.
Players automatically spawned with one on their hotbar when they spawned into the world for the first time (or re-spawned after being killed). Every time a player logged to the same server after their first spawn, a rock would appear in their inventory if they didn't have one already. It was commonly thought to be a nuisance once the player has gotten more efficient tools, aka the Hatchet or the Pickaxe, and was usually thrown away.
The reason why players spawned with the Rock upon re-logging (if they did't have one already), is due to highly geared players regularly forcing newly spawned players to give them their rock in exchange for their life. This left new spawns defenseless, and extremely difficult for said players to gather resources. After observing this behavior, the developers updated the game so you would spawn with a rock each time you re-log on the same server to ensure that players always had some way to gather resources.
- In one of Markiplier's Rust videos, both he and Bob refer to the Rock as the 'Gun Rock'. This was a reference to the fact that when a player held the Rock it looked like they were holding a gun when viewed by another player.
- The rocks that the Furnace was made out of appear to resemble the handheld Rock. However, it has not been confirmed whether it is the same view model or not.