How do you get the Gemstone Stocking Icon + Gemstone?
It was sold in the League of Legends store. Riot's description in the client reads: “This icon was released in the store for purchase during the 2017 Snowdown event.”
Is the Gemstone Stocking Icon + Gemstone still available?
Probably not. Riot tied the Gemstone Stocking Icon + Gemstone to 2017 (store purchase), and it does not normally bring back past event rewards or bundles.
