Coin
As the name implies, gold coins are coins made of gold. They are generally used as currency in the Mushroom Kingdom.
In the RPG games, coins could be used to purchase various things, such as advice, items, etc., and could be found in item blocks or earned by winning battles or selling items.
In most of the Super Mario Bros. and Super Mario World games, 100 Gold Coins can be traded in for an extra life.
In Super Mario 64, 100 of these coins will grant Mario a secret Power Star.
In Super Mario Sunshine, 100 of these coins will grant Mario a Shine Sprite. 50 of them will give him an extra life.
In Super Mario RPG: Legend of the Seven Stars, coins are used as currency to purchase services and items throughout the Mushroom World. Small coins are worth one coin while large coins are worth ten coins.
In Mario & Luigi: Superstar Saga, in the Beanbean Kingdom, they are known as Mushroom Coins and are worth far less than Beanbean Coins.
In Luigi's Mansion, the gold coin is the least valuable treasure, accounting for only 5,000G per coin. Luigi loses a few gold coins when he takes damage.
Gold coins are used as currency to buy items (including a Star) in the Mario Party series. You collect these coins by landing on coin squares, winning them after mini-games, winning the lottery, etc. At the end of the Mario Party tournament there is a chance you will recieve a bonus Star if you have the most coins out of all the contestants.
