When it comes to gemming, enchanting and reforging; I can barely work out what is OK for my own spec, let alone anyone else. So when in doubt, I ask Mr Robot. On top of that I am trying to work out what gem cuts to buy next for my JC.
Today, I looked up the PVE recommended gems for various specs with (apparently) best in slot gear.
I won't pass much judgement about the theory crafting behind some of the decisions, though I am sometimes confused (Pally tanks value dodge & parry more than mastery - yet are recommended pure mastery gems? Diminishing returns?)
I was specifically looking for recommended Dream Emerald cuts. There is 1 (Puissant - Mastery/Stamina); There is also only 1 recommended Amberjewel (Fractured - Mastery).
I have not looked at any PVP gem cuts, unsure where PVP players get their recomendations.
Players wear all sorts of gems, and get their advice (or lack thereof) from lots of different places.
Some even think any epic gems must be better than any rare gems (they are not - the epics are northrend and carry less bonus than a cata rare), so don't restrict your sales to this list.
However, being aware of what some knowledgeable players will be picking up is still a good idea. My consolidated list of gems as recommended by Ask Mr Robot for BIS Tier 12 is here on google docs.