`And you, Ring-bearer,' she said, turning to Frodo. `I come to you last who are not last in my thoughts. For you I have prepared this.' She held up a small crystal phial: it glittered as she moved it, and rays of white light sprang from her hand. 'In this phial,' she said, `is caught the light of Eärendil's star, set amid the waters of my fountain. It will shine still brighter when night is about you. May it be a light to you in dark places, when all other lights go out. Remember Galadriel and her Mirror! '

(LoTR, I-8)

It took his hand, and as Frodo watched with his mind, not willing it but in suspense (as if he looked on some old story far away), it moved the hand inch by inch towards the chain upon his neck. Then his own will stirred; slowly it forced the hand back. and set it to find another thing, a thing lying hidden near his breast. Cold and hard it seemed as his grip closed on it: the phial of Galadriel, so long treasured, and almost forgotten till that hour. As he touched it, for a while all thought of the Ring was banished from his mind.
...
He took his staff in one hand and the phial in his other. When he saw that the clear light was already welling through his fingers, he thrust it into his bosom and held it against his heart.

(LoTR, IV-8)

'Master, master!' cried Sam, and the life and urgency came back into his voice. 'The Lady's gift!
`The star-glass?' muttered Frodo, as one answering out of sleep, hardly comprehending. `Why yes! Why had I forgotten it? A light when all other lights go out! And now indeed light alone can help us.'
Slowly his hand went to his bosom, and slowly he held aloft the Phial of Galadriel. For a moment it glimmered, faint as a rising star struggling in heavy earthward mists, and then as its power waxed, and hope grew in Frodo's mind, it began to burn, and kindled to a silver flame, a minute heart of dazzling light, as though Eärendil had himself come down from the high sunset paths with the last Silmaril upon his brow.
The darkness receded from it until it seemed to shine in the centre of a globe of airy crystal, and the hand that held it sparkled with white fire.
Frodo gazed in wonder at this marvellous gift that he had so long carried, not guessing its full worth and potency. Seldom had he remembered it on the road, until they came to Morgul Vale, and never had he used it for fear of its revealing light.

(LoTR, IV-9)

Slowly the eyes crept nearer.
`Galadriel! ' he called, and gathering his courage he lifted up the Phial once more.
The eyes halted. For a moment their regard relaxed, as if some hint of doubt troubled them. Then Frodo's heart flamed within him, and without thinking what he did, whether it was folly or despair or courage, he took the Phial in his left hand, and with his right hand drew his sword. Sting flashed out, and the sharp elven-blade sparkled in the silver light, but at its edges a blue fire flicked. Then holding the star aloft and the bright sword advanced, Frodo, hobbit of the Shire, walked steadily down to meet the eyes.
They wavered. Doubt came into them as the light approached. One by one they dimmed, and slowly they drew back. No brightness so deadly had ever afflicted them before. From sun and moon and star they had been safe underground, but now a star had descended into the very earth. Still it approached, and the eyes began to quail. One by one they all went dark; they turned away, and a great bulk, beyond the light's reach, heaved its huge shadow in between. They were gone.

(LoTR, IV-9)

Even as Sam himself crouched, looking at her, seeing his death in her eyes, a thought came to him, as if some remote voice had spoken. and he fumbled in his breast with his left hand, and found what he sought: cold and hard and solid it seemed to his touch in a phantom world of horror, the Phial of Galadriel.
'Galadriel! ' he said faintly, and then he heard voices far off but clear: the crying of the Elves as they walked under the stars in the beloved shadows of the Shire, and the music of the Elves as it came through his sleep in the Hall of Fire in the house of Elrond.
...
As if his indomitable spirit had set its potency in motion, the glass blazed suddenly like a white torch in his hand. It flamed like a star that leaping from the firmament sears the dark air with intolerable light. No such terror out of heaven had ever burned in Shelob's face before. The beams of it entered into her wounded head and scored it with unbearable pain, and the dreadful infection of light spread from eye to eye. She fell back beating the air with her forelegs, her sight blasted by inner lightnings, her mind in agony.

(LoTR, IV-10)

`I do, Mr. Frodo. Hold up now! Its the gate. Theres some devilry there. But I got through, and Im going to get out. It cant be more dangerous than before. Now for it!
Sam drew out the elven-glass of Galadriel again. As if to do honour to his hardihood, and to grace with splendour his faithful brown hobbit-hand that had done such deeds, the phial blazed forth suddenly, so that all the shadowy court was lit with a dazzling radiance like lightning; but it remained steady and did not pass.
Gilthoniel, A Elbereth! Sam cried. For, why he did not know, his thought sprang back suddenly to the Elves in the Shire, and the song that drove away the Black Rider in the trees.
Aiya elenion ancalima! cried Frodo once again behind him.
The will of the Watchers was broken with a suddenness like the snapping of a cord, and Frodo and Sam stumbled forward. Then they ran. Through the gate and past the great seated figures with their glittering eyes. There was a crack. The keystone of the arch crashed almost on their heels, and the wall above crumbled, and fell in ruin.

(LoTR, VI-1)

At first he could see nothing. In his great need he drew out once more the phial of Galadriel, but it was pale and cold in his trembling hand and threw no light into that stifling dark. He was come to the heart of the realm of Sauron and the forges of his ancient might, greatest in Middle-earth; all other powers were here subdued.

(LoTR, VI-3)