A Word on Scaling Gems
I keep getting asked about these, because they are noticeably missing from all of my guides. As of 1.07, and currently in 1.09 Cold gems from Pthumerian Elder, which once gave up to +22.5 ARC Scaling, now give up to +65. Many people are excited about this, and quite understandably so, as it makes them best-in-slot for two of your gems in nearly all circumstances (Kos Parasite is a notable exception). For the 3rd slot, simply use a powerful percentage gem from one of the lists above.
However, it's my opinion that this absurd increase in power is the result of a bug. Here's why:
Only Cursed Cold gems that can drop from Depth 5 Pthumerian Elder, and Cursed Heavy gems that can drop from Headless Bloodletting Beast were increased.
Heavy Gems from Watchers, Cold gems from anyone but Elder, and all Sharp gems are unaffected.
Elder's gems should give less Scaling than Headless BLB's, due to having a secondary effect and reduced Curses. Prior to 1.07 they did (22.5 vs 24.8), but with this glitch they do not. Both give up to +65.
Both of these gems have secondary bonuses, which for every other gem means their values should be lower than the corresponding Watcher gem, even comparing rating 20 to rating 19, but this is not the case. No other gems are like this.
Because of these glaring inconsistencies, and chiefly the fact that only these two sources, and only the Cursed gems rating 18, 19, and 20 are affected, leads me to the conclusion that buffing scaling gems was not an intended result of the patch. If that were the case, why would so few have been buffed?