The Spin wheel was updated in 2.2.9 based on player feedback. It is more fair than the old version as all positions are now roughly equally likely to win (and thus rewards will not skewed by prizes of lower grade). There are 11 known prizes and one secret randomized Grand Prize.
How to Play
To spin the wheel, swipe quickly. You can set up the starting position before spin by swiping slowly.
You get one free spin every day. You can spin it an additional time by watching a video (once per day), or paying 2 .
There are 12 prizes, their position is shuffled before every spin. Values for and scale by player Level.
There is no apparent difference in results depending on the direction of spin, or how fast you try to spin it; the Wheel always spins at the same initial speed.
Old Version - Before 2.2.9
The Spin Wheel was introduced in update 2.1.8, along with new daily login bonuses, a revamped memory game, and the ability to place paths under monsters, structures, etc. It replaced the scratch ticket. It allowed players to spin it, winning prizes such as relics, coins, diamonds, keys, and food. Players could use 2 diamonds to spin it again after they’ve spun it once already that day.
The prizes that could be earned from the Spin Wheel are the same as the Scratch Ticket.
Notes on old version
It took about 10 seconds for the Spin Wheel to generate a prize. In comparison to the Scratch Ticket, it was significantly longer, since the Scratch Ticket could be scratched at any pace.
The chance of winning any given prize was not one in eight, as the eight wedges might imply to someone seeing it. This spin wheel did not behave like that. The probability of winning the better prizes was far lower than 1/8th, so you wouldn't expect to win big frequently! The smaller the prize the more likely it would come up for you. For example, see: reddit video post: "Is (ahem: was) the Spin Wheel Rigged?