That light was the light of the two trees that lit the Undying Lands across the sea before the sun and moon were made. Fëanor made three such jewels ere the trees were destroyed by the Great Enemy. The Silmarils they were called. But the Enemy stole them, and Fëanor and his sons took a terrible oath that they would do all in their power to retrieve them, whatever it may take.

The Enemy wore the Silmarils in his crown, for they caused all who possessed them to desire them above all things. One day, there came a Man--a human--to the Kingdom of Doriath, and he fell in love with the king's daughter there. Her name was Lúthien, and she was the fairest creature ever to be brought into the world. But her father had no love for humans, and as he had promised his daughter he would not slay the intruder, he bade him retrieve a Silmaril for a bride-price in the assumption that he would abandon hope or fail. But Lúthien abandoned her people and went with Beren her beloved, and together through much pain and sorrow they achieved the Quest.

I have not given the tale any justice, of course. But it is long, and filled with sorrow. From Beren and Lúthien who toiled for the jewel Dior was born, and from him and Nimloth came Elwing. Into Elwing's care came the Silmaril for which her grandmother and grandfather had made such a sacrifice. That is what was special about the jewel.