They're pieces that can be placed onto the portal, along with your Skylanders. I don't think it's been revealed as to what they specifically do. Judging from past packs, it's safe to assume that they will add some sort of helpful item to accompany you for a set amount of time per level. For instance, the Battle Hammer may bring up a hammer in game for, let's say 45 seconds, to help you defeat enemies.
That's right! And the Sky Diamond will drop a gem worth 20 gold for every enemy you defeat while it's on the portal. (I've heard something about training dummies + Sky Diamond = Tons and Tons of cash. Not sure if it works for them but might wanna try it :D )
Sky Diamond + Training Dummies = very quick cash. I am getting between 2000 and 5000 per character per use. A quick trip to the swap zone hub and then back to Woodborough takes seconds at which point I can use the Diamond again. Easily the most valuable magic item for helping purchase your Skylander's abilities. Wish I had known about it earlier.