The great jewel containing the light of the Two Trees in J.R.R. Tolkien’s The Silmarillion is a Silmaril.
The Silmarils are three jewels made by Fëanor, the greatest craftsman among the Ñoldor. They contain the mingled light of Telperion and Laurelin, the Two Trees of Valinor, which were destroyed by the monstrous spider Ungoliant and the rebel Vala Melkor.
After the theft of the jewels, Fëanor and his sons swear an oath to recover them. Their pursuit drives much of the tragedy of the First Age, including wars among Elves, Men, and Morgoth. The three Silmarils eventually pass through vastly different fates: one is carried into the sky, one into the sea, and one into the earth.
The singular answer is Silmaril; Silmarils is the plural. The Arkenstone belongs to The Hobbit, while the One Ring and Elessar are important objects from later Middle-earth stories.