Symbolism, Imagery, Allegory
When Thor must disguise himself as Freyja, he wears lots of bling, but one piece of jewelry really makes his disguise authentic: the necklace Brisingamen. Freyja was so obsessed with possessing the Brisingamen necklace that she slept with four dwarves to get it, which, as you can imagine, really irked her lover, Odin. He took it from her, and she had to start a war between two human tribes to get it back.
Freyja's willingness to lend her special necklace to Thor symbolizes her complete loyalty to the Aesir gods even more than her willingness to lend her feather dress to Loki. Because the necklace was so famously associated with Freyja in Norse mythology, it also symbolizes Freyja herself.
Freyja's desperate desire for Brisingamen was probably due to what jewelry symbolized more generally in Germanic culture: beauty, femininity, and, most importantly, power. A warlord's ability to dress his lady in tons of bling sent a message to his underlings that he was the man – he had won a lot of battles and pillaged a ton of gold. That's why the women in Beowulf are constantly described as "gold-adorned." Jewelry's association with power takes on its most famous incarnation in J.R.R. Tolkien's Germanic literature-inspired epic The Lord of the Rings, in which the fate of the world depends on a single ring.
Freyja, however, doesn't depend on Odin's power for the Brisingamen. In fact, she directly defies him by sleeping with other men, so that Brisingamen comes to represent Freyja's independence and power, rather than her lord's. Some jewelry that's more closely-related to a women's power and choices, rather than men's, appears in Guy de Maupaussant's short story "The Necklace," and George Eliot's Daniel Deronda. In these stories, women's pride and choices become interestingly entangled in the jewelry they wear.