Silmarils

Quenya: Stem: sil ... white flame sing.: Silmaril, pl.: Silmarilli, westronized pl. Silmarils

The Silmarils were three supreme jewels created by Fëanor son of Finwë. They captured the Light of the TwoTrees, Telperion and Laurelin. The creation of these jewels was the pinnacle of of Fëanor's power as a craftsman. They were stolen by Melkor, whom Fëanor named Morgoth. This theft caused the Noldorin elves to pursue Morgoth out of Valinor to Middle-earth.

Then Fëanor swore a terrible oath. His sons leapt srightaway to his side and took the selfsame vow together, and red as blood shone their drawn swords in the glare of the torches. They swore an oath which none shall break, and none should take, by the name even of Ilúvatar, calling the Everlasting Dark upon them if they kept it not; and Manwë they named in witness and Varda, and the hallowed mountaim of Taniquetil, vowing to pursue with vengeance and hatered to the ends of the World Vala, Demon Elf or Man as yet unborn, or take or keep a Silmaril from their possession. The Silmarillion, Of the Flight of the Noldor

..Morgoth forged for himself a great crown of iron, and he called himself King of the World. In token of this he set the Silmarils in his crown. The Silmarillion, Of the Flight of the Noldor

Thingol later had the Silmaril set in Nauglamír the Necklace of the Dwarves

Morgoth held onto the remaining two Silmarils until his defeat in the War of Wrath. Eönwë took them and guarded them.

Maedhros and Maglor, The surviving {Sons of Fëanor}?, they disguised thenselves, and came in the night to the camp of Eönwë, and crept into the place where the Silmarils were kept and slew the guards. The camp raised the alarm and the brotheers prepared to fight to the death, but Eönwë premitted them to escape.

But the Silmarils burn their hands and, they knew that their oaths were in vain and, that their claims were void.

Maedhros in anguish and pain cast himself and his Silmaril into a chasm of fire. Maglor in final torrment cast his Silmaril into the sea and and wandered upon the shores singing in pain and regret.

So the final resting place of the Silmarils was one to the Heart of the Earth, one to the Depths of the Sea, and the last one into the Air of the Heavens.

(The Tale of the Last Silmaril is found here; The Necklace of the Dwarves)

Quotes:

But the chief artificer of the Elves (Fëanor) had imprisoned the light of Valinor in the three supreme jewels, the Silmarilli, before the Trees were sullied or slain. The Letter's of J.R.R. Tolkien, Letter #131 to Milton Waldman

...he [Beren] penetrates the stronghold of the Enemy and wrests one of the Silmarilli from the iron crown. Ibid.

Lúthien was spent, and she had not the time nor strength to quell the the wolf. But Beren strode forth before her, and in his right hand he held aloft the Silmaril. Carcharoth halted, for a moment afraid, Get you gone, and fly! cried Beren: for here is the fire that shall consume you, and all evil things. And he thrust the Silmaril before the eyes of the wolf. The Silmarillion, Of Lúthien and Beren