On Choosing your stats
In theory, gems can be a place to switch up your stat choices and tweak and customize your armor. In practice, it's generally a good idea to choose stats that are in line with your spec. For DPS, your priority for gemming should probably look something like this: Hit to 8%>Strength>Expertise to dodge cap>critical strike rating>Everything else.
As we've discussed before, 8% hit is the bare minimum you need to prevent misses with 2-handed weapons and melee/physical special attacks. For some classes, such as feral druids and hunters, this makes it easy. Just hit that 8%, and you're golden. It's a bit more complicated for us, since for preventing misses with spells and dual wielding, you need a whole lot more hit.
Luckily, a lot of Death Knight raid parses are suggesting that 8% may be all you really need. Once you hit that, strength seems to provide an overall better dps buff. So if you want to experiment a bit, you may find that 8% hit is all you need to stay competitive on the damage meters.
Expertise is a bit more sticky. It affects only melee attacks, and maybe Death Knight DPS specs end up doing most of their damage through spell-like effects, which don't need expertise at all. We also get a lot of our expertise through talents, which helps as well. Still, if your damage feels a bit lackluster and you seem to do most of your damage via weapon strikes and melee, you may want to grab that extra 21 expertise skill or so to hit the dodge cap and see what it does for you. Otherwise, it's hard to go wrong gemming strength. Critical Strike rating has its place too, especially if you have talents that trigger on critical strike.
For Tanks, you'll definitely want to gem for defense first up to that 540 skill cap. After that, it widely depends on your gear and needs. If you're having trouble surviving, go for stamina gems, and maybe some parry or dodge. If you need more threat, strength provides and some bonus parry, but expertise might prove better since it prevents parries as well, thus giving you less change to take an extra parry-hasted hit.
In final, general advice, while it may be tempting to gem to grab set bonuses, always make sure the set bonus is worth it. If the bonus is armor penetration, as of 3.0.8, it's probably not worth it. A +3 strength socket bonus may look nice, but if it means you have to settle for a purple gem instead of a red one, the stat tradeoff may be worse after all -- unless, of course, that purple gem helps you make your metagem bonus, in which case it all becomes a nice synergy. In short, be sure to
Finally, don't be afraid to splurge on gems when it comes to your raiding set. If you have a piece of gear that you'll take into Naxxramas with you, and you can find a blue quality gem for it, grab it. Not every blue quality gem has been implemented yet for some reason, but most of the important ones have. If you want to save a little money, you can also go for the "Perfect" green quality version.
Now, on to the individual choices for each color.
Bold gems are going to be the mainstay for Death Knight DPS gems, since they give strength, our main DPS stat. If you're looking to hit up some expertise, you can also go for Precise gems. Tanks looking to fill in a red gem slot who need some more survivability may like the parry rating on Flashing gems.
Yellow is a color you'll want to be familiar with if you need to finish off a few key stats, name hit rating (via Rigid gems) and defense rating (via Thick gems). DPS who are fine on hit rating may want to grab a Smooth gem for a nice chunk of critical strike rating.
Blue's options are sort of limited for Death Knights. For tanks, though, you can rarely go wrong with a nice Solid gem for a stamina boost. DPS don't really have a blue option, and should probably go for green or purple colored gems to round out a metagem or slot bonus requirement if possible.
Green actually offers a pretty nice choice for tanks in the Enduring gems, which are great if you need just a little bit more to reach the defense cap before moving on to gem stamina. For DPS, green gems aren't the best since all the useful ones contain stamina, but for those DPS who need a shot of blue for a metagem or slot requirement and a bit more hit rating, Vivid Gems may fit the bill. If you don't need the hit rating, Jagged gems provide critical strike rating with their stamina.
Orange is just loaded with potentially delicious combos for DPS, allowing them to finish off their hit rating and expertise requirements without wasting stats.
Accurate gems allow you to grab some hit rating and expertise at once. Etched gems lets you finish off hit rating and move on to strength. Inscribed gems offer strength and critical strike rating if you feel you need some more balance in your DPS.
On the tank side, Orange gems are a great way to finish off your defense requirements while moving on to other stats. If you need more threat, Champion's Gems give strength and defense rating, while Resolute gems give expertise and defense rating. If you prefer to stick to defensive stats, Glimmering gems give Parry and defense, while Stalwart gems give dodge and defense.
Purple gems are another type that work well for tanks, allowing you to take a just a bit of extra defensive stats along with stamina, such as in the case of the Regal or Defender's Gems, which grant dodge and parry rating respectively.
For DPS who need to fill a blue gem slot, purple offers some options in the Guardian's gem, which provides expertise (which tanks may also want to consider if they need some extra threat), and the Sovereign gems, which provide strength.
For Death Knight tanks, stick with the Austere Earthsiege Diamond. We can use both the extra armor and stamina, whereas the other major tank option, the Eternal Earthsiege Diamond, has useless shield block, and probably shouldn't be used unless you're absolutely desperate for defense.
Chaotic Skyflare Diamond is a decent option for the Death Knight simply because that 3% extra critical strike damage isn't to be underestimated. However, you may also want to strongly consider the Swift Skyflare Diamond or the Invigorating Earthsiege Diamond, since both provide a good chunk of attack power. In the end, it probably depends on how critical strike dependent your DPS is as to which one you take.