#8 - Oct. 11, 2013, 3:45 a.m.
Deciding between a ruby or emerald is a really good question that comes up every so often, but the answer can be kinda complicated. Personally, I tend to use a ruby while leveling, and then change it up based on my actual damage range, critical hit chance, and critical hit damage once I hit 60.
However, here are two great guides:
Marquise Ruby vs Emerald Theorycrafting and
Community Commentary: Rubies, The More You Know that can help you decide what gem is best for you to use. Good hunting, and good luck!