Faction specific. Maybe I just got lucky, but I got way more rares than commons.
Commons have a light border, uncommons have a darker border, and rares have an even darker border and some cracks on the card.
Yeah, I usually tell rarity by the number of cuts on the side of the card. If you open up a 10-scroll booster, the last three picks will be uncommon, uncommon, rare. That should give you a chance to see how the border differs for each type. I think another GAFfer said that there are more colored stripes depending on rarity - one for common, two for UC, and three for rare.