CryptoSkulls — one of the first NFTs! CryptoSkulls are 10,000 collectible ERC-721 Non-Fungible Tokens stored on Ethereum and Polygon (previously Matic) blockchains. Each CryptoSkull character is a unique generated pixel image with Uniqueness Index property. That property shows total rareness of all image traits. CryptoSkulls with rarer features have less Uniqueness Index. Since 2019.