Freyja's necklace made by the fairies. Freyja left her husband Odin in order to obtain this necklace; and Odin deserted her because her love was changed into vanity. It is not possible to love Brisingamen and Odin too, for no one can serve two masters.
As a moral tale this is excellent. If Freyja personifies “the beauty of the year,” then the necklace means the rich autumn tints and flowers, which (soon as Freyja puts on) her husband leaves her- that is, the fertility of the genial year is gone away, and winter is at hand.