Crown Gems are spent within the under the Crown Crates tab. Each crate "season" has its own dedicated rewards list where you can use Gems to buy:
How to get free Crown Crate items // Beginner's Guide to ESO eso buy crown gems
: You can manually deconstruct unwanted consumables obtained from crates—such as potions, poisons, and experience scrolls—in the Gem Extraction menu. Crown Gems are spent within the under the Crown Crates tab