I doubt that it protected Reen from Vin; he wouldn't have left it with her if that was the case because it would have rendered her helpless. At first glance, I would say that the obsidian isn't noteworthy, I mean how does Reen, a member of a thieving crew, acquire this all important relic let alone Vin not know about it's importance. Then I go back and look at some of these quotes that we posted and it brings up questions about Vin and her family.
a pebble from each city she'd visited, the earring Reen said Vin's mother had given her, and a bit of obsidian the size of a large coin. It was chipped into an irregular pattern - Reen had carried it as some kind of good luck charm. It was the only thing he'd left behind when he'd snuck away from the crew half a year before.
Tevidian was visibly shaking. “I … I think I got them all, my lord. There … there was one I may have been too lax with. I didn’t know she was skaa at first.
First, Vin has traveled to other cities besides Luthadel. I don't get the impression that a thieving crew would travel great distances when Luthadel is ripe for the taking. When Vin first leaves the city with Kelsier it's almost as if it was the first time she had gone outside the walls, except we know by the first quote that she had seen other cities. So that leads me to the conclusion that she had traveled when she was young, probably when her mother was alive because I just can't see Reen traveling alone with a very young Vin.
The second quote, Tevidian says he didn't know she was a skaa at first. Now, he could have been lying to save his life or he could have been telling the truth. If he was telling the truth, how does a Skaa pull off such a trick. We saw what it took for Kelsier and crew to get Vin infiltrated into their society and he had a very large source of funds. How did Vin's mother pull it off?
So we see that Reen and Vin have traveled outside of Luthadel and Vin's mother was able to seduce not just an Obligator, but one that had VERY good bloodlines. Reen walks around with this obsidian like it is his good luck charm and one day leaves it for Vin without ever returning because he was unable to avoid the Inquisitors for the first time. I would think that anything to do with Vin's mother and Reen need to be carefully examined because none of it looks like mere coincidence.
I have alot more to theorize, but I have been up a very long time and we still got a couple months to formulate and reformulate.
