Gemstones and Prestige points were two of the rarest items in League of Legends, allowing them to purchase unique skins that you couldn't find on the regular shop. However, during season 2022, League of Legends will retire both those items and introduce a new rare currency called Mythic essence.
This new currency can be used to buy both prestige skins as well as Hextech skins within the game from season 2022 onwards. Hextech skins are hard to obtain, and will only be earnable through the new Mythic essence currency.
During the 2022 season, the brand new mythic essence shop will feature rotating prestige skins. Every month, an old prestige skin will feature in the shop replacing the previous one. From this season onwards, League of Legends will introduce seasonal mythics. Every season, there will be a theme based on which the skins will be developed.
The Mythic essence can be obtained in the exact same method through which gemstones and prestige points are earned. While prestige points that any player has will soon expire, the gemstones that each player has will convert to Mythic essence.