Gems are the premium currency in Nonstop Knight2. They can be used to buy coins, shards, items, and bundles. You can earn small amounts of gems by meeting objectives, watching ads for example. You can also purchase them in the shop for real money.
Coins are a type of currency used to buy and empower items.
Shards are a type of elemental currency that is used to empower items of the same elemental type. These are Fire Shards, Metal Shards(for non-elemental items), Frost Shards, and Void Shards.
Silver Keys are a type of currency that allows you to open silver chests obtained at the end of a campaign node.
Royal Tokens are a type of currency used to empower and recharge items(for further empowerment).