Possible Duplicate:
Should I socket my weapon with a Ruby or Emerald?
At about level 20 you start getting weapons with slots. First it is way more powerful to insert a ruby for the +X damage.
Later as the weapon damage increases the benefit from rubies get less and it is way better to insert an emerald for the +X% critical hit damage.
Is there a rule or guideline how to determine the best point to switch instead of trying both gems?
I am not certain that this goes for every class, as I have only a high level Demonhunter.