The Yellow Gem is a yellow emerald cut gem, similar in appearance to a Sol Emerald, that can be equipped on Sonic's Custom Shoes. When used, it allows Sonic to create the Thunder Guard, a type of yellow Lightning Shield. If the Thunder Guard shield is destroyed by enemy attacks or obstacles, it can simply be put up again by using the Yellow Gem again, allowing a near permanent shield.
While having set Sonic's Custom Shoes to utilize the Yellow Gem's ability, the Yellow Gem changes their appearance into a yellow pair of shoes resembling Sonic's 2G Hi-Speed Shoes.
- When the Thunder Guard has been activated, it will remain active, even if the player changes the Yellow Gem Level Up Item to a different Gem.
- Official In-Game Description