The Prose Edda 2.33

Wherefore is gold called Ægir's Fire? This tale is to the same purport as we have told before: Ægir went to Ásgard to a feast, but when he was ready to return home, he invited Odin and all the Æsir to visit him in three months' time. First came Odin and Njördr, Freyr, Týr, Bragi, Vídarr, Loki; likewise the Ásynjur: Frigg, Freyja, Gefjun, Skadi, Idunn, Sif. Thor was not there, having gone into the eastern lands to slay trolls. When the gods had sat down in their places, straightway Ægir had bright gold brought in onto the floor of the hall, and the gold gave forth light and illumined the hall like fire: and it was used there for lights at his banquet, even as in Valhall swords were used in place of fire. Then Loki bandied sharp words with all the gods, and slew one of Ægir's thralls, him who was called Five-Finger; another of his thralls was named Fire-Kindler. Rán is the name of Ægir's wife, and their daughters are nine, even as we have written before. At this feast all things were self-served, both food and ale, and all implements needful to the feast. Then the Æsir became aware that Rán had that net wherein she was wont to catch all men who go upon the sea. Now this tale is to show whence it comes that gold is called Fire or Light or Brightness of Ægir, of Rán, or of Ægir's daughters; and now such use is made of these metaphors that gold is called Fire of the Sea, and of all names of the sea, even as Ægir or Rán had names associated with the sea. Therefore gold is now called Fire of Waters or of Rivers, and of all river-names.

"But these names have fared just as other figures also have done: the later skalds have composed after the examples of the old skalds, even those examples which stood in their poems, but were later expanded into such forms as seemed to later poets to be like what was written before: as a lake is to the sea, or the river to the lake, or the brook to the river. Therefore all these are called new figures, when terms are expanded to greater length than what was recorded before; and all this seems well and good, so fair as it concurs with verisimilitude and nature. As Bragi the Skald sang:

I was given by the Battler The fire of the Brook of Sea-Fish: He gave it me, with mercy, For the Drink of the Mountain-Giant.