A kindjal is a double-edged Circassian dagger.
History
Valentine Morgenstern once inherited a pair of kindjals from his father. They each had a star, the symbol of the Morgensterns, carved into the blade. The longer of the pair had blue stones on its hilt, and the smaller had a single red stone in the shape of a rose.
When Valentine's parabatai Lucian was bitten by a werewolf and subsequently Turned, Valentine gave him the blue-studded kindjal and encouraged him to use it to kill himself. Though Luke did not use it in the way Valentine intended, he decided to keep the blade and has since used it on several occasions.
Later, Valentine gave the other kindjal to Jace, whom he led to believe was his biological son. Jace then lent it to Clary Fray, who temporarily lost it after using it to attack Alaric at the Hotel Dumort, not knowing that he was in Luke's werewolf pack. Alaric later returned it to her, but she eventually gave it back to Jace.[1]