Ruby

Rubies are the premium currency in Mario Kart Tour mainly used to summon new drivers, karts, and gliders from the pipe. Five are required to fire it one time, although 45 can be spent to fire 10 at once. If the player does not have any or enough Coin Rush tickets, rubies can be spent to play Coin Rush, with 5 rubies multiplying the coin count by 2, 15 by 6, and 25 by 10. In addition to being purchasable with real money, rubies are obtainable in-game through Tour Gifts, by completing the game's standard challenges, as an occasional daily reward, and as a reward for reaching certain levels.

In-game description
"You can use these to fire off the pipe or play Coin Rush."