He saw on a sudden a slender sheen
shine a-shimmering in the shades afar,
like a glow-worm's lamp a-gleaming dim.
He marvelled what it might be as he moved softly;
for he knew not the Gnomes of need delving
in the deep dungeons of dark Morgoth.
Unmatched their magic in metal-working,
who jewels and gems that rejoiced the Gods
aforetime fashioned, when they freedom held,
now sinking slaves of ceaseless labour
in Angband's smithies, nor ever were suffered
to wander away, warded always.
But little lanterns of lucent crystal
and silver cold with subtlest cunning
they strangely fashioned, and steadfast a flame
burnt unblinking there blue and pale,
unquenched for ever. The craft that lit them
was the jewel-makers' most jealous secret.
Not Morgoth's might, nor meed or torment
them vowed, availed to reveal that lore;
yet lights and lamps of living radiance,
many and magical, they made for him.
No dark could dim them the deeps wandering;
whose lode they lit was lost seldom
in groundless grot, or gulfs far under.

(HoME III, The Lays of Beleriand, Túrin Son of Húrin & Glorund the Dragon, 775-799)

And then Gelmir brought forth one of those lamps for which the Noldor were renowned; for they were made of old in Valinor, and neither wind nor water could quench them, and when they were unhooded they sent forth a clear blue light from a flame imprisoned in white crystal. [2]

[2] The blue-shining lamps of the Noldorin Elves are referred to elsewhere, though they do not appear in the published text of The Silmarillion. In earlier versions of the tale of Túrin Gwindor, the Elf of Nargothrond who escaped from Angband and was found by Beleg in the forest of Taur-nu-Fuin, possessed one of these lamps (it can be seen in my father's painting of that meeting, see Pictures by J. R. R. Tolkien, 1979, no.37); and it was the overturning and uncovering of Gwindor's lamp so that its light shone out that showed Túrin the face of Beleg whom he had killed. In a note on the story of Gwindor they are called "Fëanorian lamps," of which the Noldor themselves did not know the secret; and they are there described as "crystals hung in a fine chain net, the crystals being ever shining with an inner blue radiance."

(UT, OF TUOR AND HIS COMING TO GONDOLIN)