Brisingamen

Like most of the jewelry in Asgard, this necklace was created by the dwarves (In this specific case, Alfric, Berling, Dvalin, and Grer). The four dwarves demanded that Freya should spend a night with each of them as payment. No problem, Freya is, after all, the Goddess of Love.

Enter Loki. He heard of Freya's deal with the dwarves and decided to inform Odin. Odin punished Freya by forcing her to start a war between two human kings.

As many of the other (Gungnir, Mjölnir etc) items created by the dwarves, Brisingamen had magical powers - it protected the wearer from harm, a feature that the weak Freya would benefit from. Greedy Loki decided to steal the Brisingamen, entered Freya's castle Folkvang by transforming into a fly, transformed into a louse and bit her so she turned over in her sleep, allowing Loki to remove the necklace. Loki then hid the necklace at an islet called Singasten.

Word of the theft spread all over Asgard, and soon reached Heimdall, who immediately suspected the trickster Loki. Heimdall travelled to Singasten, located Loki - who had transformed into a seal, tranformed into a seal himself and challenged Loki. Great Old Norse Epic Battle followed. Heimdall defeated Loki and brought the Brisingamen back to its rightful owner.

An interesting follow-up to that story is that the necklace Brisingamen was responsible for Freya losing her husband Odur.

The basic plot goes something like this:

Once in Asgard gold was abundant, and all of the gods were happy. This was referred to as "The Age of Gold." Gold was so abundant that the gods built Gladsheim, the Hall of Odin out of pure gold. They also made all of their utensils out of pure gold, and they gave their finery away freely to all of the other gods.

But then, three Giant women came to Asgard, and they began to hoard and value gold. After that, the happy innocence of the gods was lost, and they began to keep the gold for themselves. The three Giant women were expelled from Asgard.

Many gods were very upset about this change in attitude, and missed the care-free good old days of plentiful gold and high spirits. One of the most vocal on this point was Odur, Freya's husband. He forbade Freya to ever go visit the Giant women and buy their finery.

Freya disobeyed her husband, and went to the mountain-top where the three Giants lived. It was there that she received the necklace Brisingamen. This necklace made her more and more beautiful, and she thought that Odur would forgive her when he saw how lovely and happy it made her. Odur left Asgard and never returned. Freya continued to wear the neckalce, no longer as a beautiful piece of craftmanship, but to show her shame that she had made the wrong decision.