The Rock of Justice was a powerful relic that existed in the Sonic Boom Zone, crafted a very long time ago to ensure that the light would always drive back the darkness...or so Sticks the Badger claimed.
After discovering the rock (after, apparently, years of searching), Sticks presented it to her friends as a weapon to defeat Dr. Eggman's fourth and final incarnation of his Big Boy robot, however they merely dismissed it as being a part of yet another Sticks conspiracy theory. However, the rock would ultimately end up bringing about the robots final downfall when Sticks hurled it, allowing it to bounce all over, causing major damage to the bot before finally striking its power source, destroying it for good. After this incredible feat, the rock had apparently used up the last of its power and, after an emotional wake, was laid to rest...in amongst Sticks' ever-growing rock collection at her burrow. (SB: #3, #4)
It's still unknown whether the Rock of Justice truly did hold some kind of mystical power or if it was, in fact, just an ordinary rock.