Can someone explain sockets and gems to me?

[deadlyfour]

Recently started playing again after being extremely casual in MoP, however now I am playing a lot more seriously. Fire mage obviously and I have been very fortunate to receive the Shard of Exodar legendary, which has a prismatic socket, along with some of my other gear having it aswell. Can someone explain what gems I should be looking for etc? I didn't see a lot of information on it in the guides but may have missed it. Thanks!

[lopstar]

Ideally u wanna sim your statweights to see the right stat to choose. U can do that on raidbots.
Otherwise if u run kindling, and have less than 55% crit u probably wanna run crit.

[z0nkey]

As Lopstar said, simming yourself should be your #1 priority.

With Shard you want to aim for even higher crit as you don't have the execute with belt (assuming you're using bracers as your other lego).

Typically you want to gem mastery unless you're below 50% crit. If you're below that threshold you may want to consider gemming crit until you get some more gear that puts you over the threshold.

good luck