Rin's Pendant

A powerful jewel magecraft left to Rin by her father.
It contained so much magical energy that it was even capable of reviving a human being on the verge of death.
This is what Rin used to help Emiya Shirou at the outset of the story.
Though it may have seemed like she was wavering back and forth about whether or not to save him for quite a while in the prologue, in reality it took her less than a second to make her decision. She sure does make snap judgments, that Rin.
Then, when Shirou was hazily tidying up after he came to, he ended up stuffing the pendant in his pocket before heading home.
As for the sequence of events that led to the pendant returning to Rin's possession, that's covered in the story.
Honestly, I was a little worried about what might happen if somebody figured it out from playing the trial version. I don't know what I would have done.

Fate/side material: Fate Encyclopedia