Obtained by doing the Mount Hyjal quest line The Molten Offensive through step 4 Filling the Moonwell which opens up the Vendor Alya Shadowstorm who sells the pattern.
Start saving up your dreamcloth and get the quest line done on your tailor ASAP when 4.2 comes out. Jewelcrafters are going to be snapping them up fast and some will buy 4 at a time.
Start buying out and stockpiling cheap, normally or even slightly above normally priced epic Spellthreads too. They should go up in price because everyone will be using their Dreamcloth to make the new gem bags. My conservative prediction is the bags will go for 5k+ (maybe a LOT more) the first week, at least 3k for several more weeks and spellthreads will jump to 1k+. It'd be a good idea to do the same with epic Leg Armors, Pristine Hide and Heavy Savage Leather too.
Cut Gems stack to 20. Note this info is datamined from the PTR. It probably will happen but you can never be certain with datamined info.
Bloodthirsty Jewelry 6 new ilevel 358 rare PvP rings and necklaces. Each requires 4 or 5 rare gems (Inferno Ruby, Ocean Sapphire or Amberjewel) and several Volatiles to craft. These will probably sell ok. If so they'll almost certainly contribute to rising Inferno Ruby prices (which should be rising anyhow due to increased demand for gemming all the new gear). They might even cause Amberjewel and Ocean Sapphire prices to rise a bit too.
Why is Jewelcrafting the only gear crafting profession that cannot craft any level 85 Epic gear? Show us some love Blizzard!
I intend to have a good stockpile of all rare gems, especially Inferno Rubies ready for the patch. Demand will increase for all gems and dramatically so for Inferno Rubies. On patch day the first thing I intend to do is buyout all the best selling gems at normal or lower prices. Then I'll cut enough to have at least a full stack of all of the 40 or so best selling cuts and 5-10 each of other cuts. I'll quickly post some gems and then go get that bag pattern on my tailor.
One other point to consider. Blizzard might decide to slip Epic Gems into 4.2 at the last minute. It's not a bad idea to stockpile some tokens to buy patterns with on patch day just in case they do. For those that don't mind taking some risk you might want to stockpile Pyrite Ore in case Epic Gems can be prospected or Rare Gems and Herbs for possible Epic Gem transmutes.