Enamels are not very difficult to repair. If a enamel jewel has been dropped and cracked you can clean the piece with a jeweler’s steamer to remove any dirt that might have made it’s way into the crack. Let it dry!! on top of the kiln works well. Then fire. Use the same temperatures yo know for the base metal of the jewel, usually around 1450 degrees, and  the firing times you know from the size of the piece. There will be an indention in the work and you will need to choose to fill with a soft firing flux that is made to be a top coat like Ninomiya N4 or sand the piece down to match the surface again.

In the initial firing I recommend checking on the piece in the kiln through time to see when the enamel flows. Especially if it is not a piece you have made. This way you will know if the enamels are  harder firing or softer firing. Then you can match what enamel  you decide to add.

I can across this in a repair of a bangle bracelet from India. The enamel was much harder than any I had. But the design of the bracelet was in cloisonne and I was able to remove all the red in the broken cell and apply new enamel.