Don't gem mastery as a feral tanking druid. Your gemming should include...
The cuts you will find yourself using are:
Delicate (Red, Agility)
Polished (Orange, Agility/Dodge)
Shifting (Purple, Agility/Stamina)
Austere (Meta, Stamina/Armor)
Simply put, any Red or Prismatic socket gets Delicate. Any yellow socket gets Polished. Any blue socket gets Shifting.
If the socket bonus is anything other than Agility or Mastery it likely isn't worth pursuing and you would prioritize Agility only gems, though most of the time these will be the socket bonuses on Agility items.
You shouldn't gem for anything other than those three stats. You may adjust your gem selection to include the Solid cut (just stamina) though you shouldn't require a stat shift like that until at least Heroic Raid content.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.