In J.R.R. Tolkien's The Silmarillion, the jewels stolen by Morgoth are the Silmarils.
The Silmarils are three radiant jewels made by Fëanor, the greatest craftsman among the Noldor. They contain the light of the Two Trees of Valinor, Telperion and Laurelin, and cannot be destroyed or reproduced. Their beauty and sacred origin make them far more than ordinary precious stones.
Morgoth steals the jewels after destroying the Two Trees with Ungoliant. Fëanor and his sons swear an oath to recover them, leading to rebellion, exile, and a long series of wars in Middle-earth. The oath also brings tragedy to the Elves and their allies, because the claim to the jewels becomes entangled with violence and betrayal.
The Silmarils are not the One Ring, the Palantíri, or the Arkenstone. The Arkenstone is a famous Dwarven gem from Erebor, while the Silmarils belong to the mythology and First Age history of Tolkien's world.