In J.R.R. Tolkien’s The Silmarillion, the Silmarils contain the light of the Two Trees of Valinor.
The three jewels were made by Fëanor, the greatest craftsman among the Noldor. He captured within them the mingled light of Telperion and Laurelin, the Two Trees that illuminated Valinor before the Sun and Moon existed. Their beauty and sacred light made them the greatest treasures of the Elves.
After Morgoth destroyed the Two Trees and stole the jewels, Fëanor and his sons swore an oath to recover them. That oath drove much of the tragic history of the First Age, including the exile of the Noldor and conflicts among Elves, Men, and other peoples.
The Silmarils were not merely decorative gems. They could not be touched safely by unclean hands, and their fate connected the beginning of the Sun and Moon with the wars against Morgoth. The Arkenstone and One Ring are separate objects.