How do you get the final Chaos Emerald in Sonic Colours for DS?

In order to get the final Chaos Emerald in Sonic Colours for DS, you need to complete all of the following tasks:

* Complete all Acts in every world

* Achieve an S Rank on every Act in every world

* Collect all 180 Red Star Rings

* Collect all 30 Big Rings

Once all of these tasks are complete, you can head back to Planet Wisp and talk to Omochao, who will give you the final Chaos Emerald.