Yellow Gem

The Yellow Gem is is an optional upgrade for Sonic the Hedgehog in Sonic the Hedgehog (2006) and is one of the seven customized Gems that can be obtained in the game. It allows Sonic to create the Thunder Guard, a type of yellow Lightning Shield, but he cannot double jump. When in use, the Yellow Gem changes the appearance and colors of Sonic's Power Sneakers into a yellow pair of new shoes.

Location
In Sonic the Hedgehog (2006) the Yellow Gem becomes available at the local shop, once the player clears Sonic's version of Kingdom Valley in the main story. It can then be bought for 12,000 Rings.