Silima

The Silima is the unknown material which Fëanor used along with the light of the Two Trees of Valinor to create the Silmarils.

"But not until the End, when Fëanor shall return who perished ere the Sun was made, and sits now in the Halls of Awaiting and comes no more among his kin; not until the Sun passes and the Moon falls, shall it be known of what substance they were made."

-J.R.R. Tolkien. The Silmarillion.

Description
It appeared like the crystal of diamonds, yet was stronger than adamant. No violence could mar it or break it in the Kingdom of Arda.