Jewels are special Gems, they can be placed in a Gem Slot.
Jewels are having limit on how many of them can be placed into a certain item. It is called "Socketability". Some Jewels can be used only in one slot and some can be used in multiple slots. Always look for the sockeability info on the jewel. In example, if it says 1/3 that means you have used one jewel out of maximum three.
Jewels can be upgraded with Shiny Dust. Just like Gems and Runes, Jewels are having tiers. At the moment they are having only 3 tiers (Common, Improved, Magic) but they will probably have more tiers in future (Common to Legendary).
Jewels can be also destroyed and turned into Shiny Dust.
In order to be able to craft Jewels at the Workbench you need to buy the Recipe.