The Race Piece holds the rights of the race they belong to, so while the Races Pieces can be bet, it is unwise to do so. If a race's representative bets their race's Piece and loses, the race's rights as an Exceed are stripped and the pledges no longer apply to them. Hence, no one has ever bet their Piece in the past making Sora and Shiro being the first ones to do so.
The Race pieces all take on the form of a piece in Chess. When a new representative for the race has been chosen, the race piece will materialize before them, as shown with Sora and Shiro. The race pieces are translucent and emit a multi-colored glow and at times exudes a sparkle as well.
It is later revealed that, by conquering all the Exceed races and thus having all the 16 chess pieces, one attains the right to challenge Tet for the title of One True God, as one side of a chessboard consists of 16 pieces. Tet did not create the concept of the race pieces, however, Riku and the "179 Ghosts" of humanity did as shown in volume 6.
Sora and Shiro were the first representatives to bet their race's Piece, and after winning, they were also the first to succeed in holding on to it.