Crown Gems cannot be purchased directly with real money or Crowns; instead, they are a byproduct of opening . They function as a "fail-safe" currency, allowing you to eventually buy specific crate items you want rather than relying solely on random drops. How to Acquire Crown Gems

You can only obtain Crown Gems through the following methods:

: 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. 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