Feyre's ring is the ring that Rhysand made Feyre retrieve from The Weaver's cottage to demonstrate that being Made allowed her to locate the Book of Breathings. The ring was given to the Weaver by Rhysand's mother so she could guard it until Rhysand's future bride or mate would have to come to retrieve it, demonstrating she was a strong enough wife for her son. Rhys sent Feyre to retrieve it because he wanted to give it to her, his mate.
Centuries before the events of the book Rhysand's mother gave her ring to the Weaver so she could keep it until her son's future bride or mate would come to retrieve it.
A Court of Mist and Fury
Rhysand sends Feyre to retrieve the ring from The Weaver's cottage to demonstrate that she can locate the Book of Breathings. Feyre is able to sense the ring and against the odds she succeeds in escaping from the Weaver with it.
It is described as a ring of twisted strands of gold and silver, flecked with pearl, and set with a stone of deepest solid blue; sapphire but different. The pale lines of a six-pointed star radiates across the round, opaque surface of the blue stone.