Every coin minted can be collected, but not all coins are collectable. There are many reason for this, but becoming a sought after coin is generally due to the rarity of the piece. If few were made, then those who have a passion for this hobby will want it. Coins that have gone out of circulation often end up being destroyed, and it makes them more valuable. Value is part of the collection of coins, and there are events that add to their value.
When a coin is commissioned, a professional designer creates the mold used to make it. Each mint is given a copy of this mold, and it is individualized for the mint if a country has more than one. Some mints will find their copy has a mistake in it, and they will recall the coins or not release them. When this occurs, the coin immediately becomes a collectible.
Rare events in the history of a country are often referred to in coins, but they are not always popular with collectors when they come out. If most of them are gathered up and turned into something else, the coin then has more value. Collectors often find these types of coins are important to them, and they seek them out. A coin that a collector’s grandfather would not look at might become one the grandchild will spend an exorbitant amount to own.
The age of a coin is one more factor that equals value, and it is generally due to the rarity of it. Older coins from the past few decades are still relatively easy to find, so they are worth less. When it is more than a century old, a coin might be only one of a handful that survives to this day. Collecting ancient coins is even rarer as they are only found in museums and shipwrecks.