If I had a weapon, equipped it to my monk and said "this gives me 500% more damage, buy it for 2b gold!" would you do so? no. Because DPS% increase is relative to your current DPS.
You are only interested in the actual hard-number DPS increase because that will be the same across the board in each case.
For some monks, a 12k damage increase would be INSANELY good.
For some, it would be negligible.
This is why I am talking about the fact that, even in your example, the Ruby was 2x more effective in the faster weapon. (200 DPS gain vs 100 DPS in weapon A)
I am not currently even talking about Emeralds. Their effect is the same for every single variable except CHC which means they do not care at all about the parameters of your weapon because it can't have CHC on it.
My only points are:
If you only have 1 weapon equipped, a ruby will be STRICTLY more effective (add more paperdoll DPS) if that weapon has a higher APS.
IE: If you have two weapons with identical DPS but one has a higher APS, your paperdoll DPS will be HIGHER if you use the faster one with a ruby in it.
Putting a Ruby in either hand of a duel-wield situation, will NOT effect your paperdoll DPS.
It is possible that, when duel-wielding, putting a Ruby in your SLOWER weapon may increase your eDPS because of Cyclone.