Draupnir Origin

Why is gold called Sif’s hair? Loki Laufeyiarson had done this
for love of mischief: he had cut off all Sif’s hair. And when Thor
found out, he caught Loki and was going to break every one of his
bones until he swore that he would get black-elves to make Sif a
head of hair out of gold that would grow like any other hair. After
this Loki went to some dwarfs called Ivaldi’s sons, and they made
the head of hair and Skidbladnir and the spear belonging to Odin
called Gungnir. Then Loki wagered his head with a dwarf called
Brokk on whether his brother Eitri would succeed in making three
precious things as good as these were. And when they got to the
workshop, Eitri put a pig’s hide in the forge and told Brokk to
blow and not stop until he took out of the forge what he had put
in. And as soon as he left the workshop and the other was
blowing, a fly settled on the latter’s arm and nibbled, but he went
on blowing as before until the smith took his work out of the
forge, and it was a boar and its bristles were of gold. Next he put
gold into the forge and told Brokk to blow and not stop the
blowing before he came back; he went out. And then the fly came
and settled on his neck and nibbled twice as hard, but he went on
blowing until the smith took from the forge a gold ring called
Draupnir.

[...]

Then Brokk brought out his precious things. He gave the
ring to Odin and said that every ninth night there would drip from
it eight rings equal to it in weight.


- Prose Edda, Skaldskaparmal