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. His hands were burned black by the touch of those hallowed jewels, and black they remained ever after; nor was he ever free from the pain of the burning, and the anger of the pain. That crown he never took from his head, though its weight became a deadly weariness.

(Silm, Chapter 9, Of the Flight of the Noldor)

...not all the power of the Noldor, before the Siege was broken, had availed even to see from afar the shining Silmarils of Fëanor. For they were set in the Iron Crown, and treasured in Angband above all wealth; and Balrogs were about them, and countless swords, and strong bars, and unassailable walls, and the dark majesty of Morgoth.

(Silm, Chapter 19, Of Beren and L�thien)

All his court were cast down in slumber, and all the fires faded and were quenched; but the Silmarils in the crown on Morgoth's head blazed forth suddenly with a radiance of white flame; and the burden of that crown and of the jewels bowed down his head, as though the world were set upon it, laden with a weight of care, of fear, and of desire, that even the will of Morgoth could not support.

(Silm, Chapter 19, Of Beren and L�thien)

Before his feet he saw amazed
the gems of Fëanor, that blazed
with white fire glistening in the crown
of Morgoth's might now fallen down.
To move that helm of iron vast
no strength he found, abd thence aghast
he strove with fingers mad to wrest
the guerdon of their hopeless quest,
till in his heart there fell the thought
of that cold morn whereon he fought
with Curufin; then from his belt
the sheathless knife he drew, and knelt,
and tried its hard edge, bitter-cold,
o'er which in Nogrod songs had rolled
of dwarvish armourers singing slow
to hammer-music long ago.
Iron as tender wood it clove
and mail as woof of loom it rove.
The claws of iron that held the gem,
it bit them through and sundered them;

(HoME III, The Lays of Beleriand, The Lay of Leithian, 4132-4151)

Then he was bound with the chain Angainor which he had worn aforetime, and his iron crown they beat into a collar for his neck, and his head was bowed upon his knees. And the two Silmarils which remained to Morgoth were taken from his crown, and they shone unsullied beneath the sky; and Eönwë took them, and guarded them.

(Silm, Chapter 24, Of the Voyage of E�rendil and the War of Wrath)