What is the mod TobyGames uses in Minecraft season 3?

In the majority of Minecraft season 3, he uses this one mod (I'm not good with Minecraft, so there's a possibility that I'm wrong about all of this, it may not even be a mod - hell, I might even be calling it the wrong thing), and I want to know what it is.
It's snowy, it has all these mountains and tons and tons of jewels and ore and things. Like I said, I'm horrid at Minecraft (hence why I watch other people play it), so please correct me if you have no idea what I'm talking about.

The mountains probably come from Biomes 'a Plenty. It is a mod that adds a lot of terrain variation to the game. ExtraBiomes XL an add-on to Biomes 'a Plenty adds even more.

Looking at his season three list, it seems like he uses and drops a lot of mods in season 3, could you be more specific on which video you see the gems in? There are 100 episodes and I don't really want to go through each one looking to see what the gem looks like.

Red Power (and its 'replacement' Project Red) add gems, as does More Ores mod and quite a few others.