Silver Coin
Silver Coins are coins that only appear when a P-Switch has been pressed, and will disappear once the P-Switch wears off. Although being called Silver coins, they are more pale blue in colour (however in Super Mario World, both the pale blue coins & actuall silver coins (pictured) appear.
Silver Coins first appeared in Super Mario Bros. 3, where they appeared out of thin air in certain spots for as long as a P-Switch was in effect. Silver Coins have the same value as standard Gold Coins, and are usually found in clusters.
In Super Mario World, pressing a Silver P-Switch would turn all enemies in Silver Coins for a limited time. However, Silver Coins worked differently from standard coins in this game; if enough were collected, they would start racking up into extra lives, similar to jumping on numerous enemies in succession. In certain levels, a trick involving hordes of enemies and a Silver P-Switch can be executed to earn dozens of extra lives.
In Diddy Kong Racing and its Nintendo DS counterpart, Diddy Kong Racing DS, silver coins can found found around certain tracks after beating a world boss and collecting several balloons. They are scattered throughout each track, sometimes hidden in hard to reach spots. Each one must be collected in all stages to complete the game. When taking the challenge, the player must grab each coin while racing and try to win first place. If they manage to do so, they will complete the track's challenge.
They appear in New Super Mario Bros., having the same role as in Super Mario Bros. 3. They sometimes appear in a trail behind Invincible Mario in a few levels, like World 7-3, and in a secret area in World 5-3. In New Super Mario Bros. Wii, they reappear, this time with a deeper blue color, like Blue Coins, and when the player collects them all in a level before they disappear, the audience cheers. They are often arranged in patterns.