Red Coin

Red Coins were first seen in Super Mario World 2: Yoshi's Island. In which, they were hidden in Gold Coins. The only way to tell which coin was which was to grab it and see or use a Magnifying Lens.

They were later see again in Super Mario 64. In which, one was worth two Gold Coins. Collecting all eight in one course would make a Power Star appear over a Star Marker.

They made another appearance in Super Mario Sunshine. This time, collecting all eight would make a Shine Sprite appear.