#Update: Masterwork chest is now active
Also note that you can get your gemstone refunded if you had cashed it in before this feature went live, just submit a ticket to Support and they should be able to sort it out for you!
(Check Riot_Mort's comment)
Riot announced (after a Reddit user proposed the change) that they were going to change the chest you get from using a Gemstone from a Hextech Chest to a Masterwork Chest.
Changes were supposed to go through in Patch 8.10 (went live this morning) yet the Hextech Chest is showing up and not the new (promised) Masterwork Chest.
Are the changes still coming or have they been delayed/cancelled?
#Shiny: Masterwork chests, which only contain cosmetics, will now be available for 1 gemstone in place of Hextech chests.