The Zeo Diamond (Zeion Crystal) is an artifact of great power, anyone who's evil can't even touch the Diamond.
Ian manages to get the Diamond, but during battle with MetallifeKuwagamon they lose the Diamond. They manage to recover it and send it back in time. After time is reversed by DarknessBagramon, the Digi-Rangers (reduced to kids; Ian, Sonia, Charlie, Adam and Mary) are sent through time to find the Zeo Diamond.
Ian is sent to Honolulu, Hawaii. Where he re-meets a friend Mizuki Amano, they use Submarimon to find Ian's Zeo Sub-Diamond. He thanks his old friend for helping out, in hope she can be a true Digi-Ranger one day.
Sonia is sent to Tokyo, Japan circa 1880s. She meets some of her heritage, and finds her Zeo Sub-Diamond, where her great-great-great-great grandparents got married.
Charlie is sent to London, England. He finds his Zeo Sub-Diamond where the 2012 Olympic Games were being held.
Adam is sent to Hong Kong, China. He meets his a relative, who helps him find his Zeo Sub-Diamond. He ends up finding out that he's a relative of Ian and Dan. (That part is in the Dub Only)
Mary is sent to the wilderness of Africa, circa 1980s. She meets a girl named Kenya Alon. After Mary retrieves her Zeo Sub-Diamond, she gives it to Kenya.
The Zeo Diamond plays a role in the entire season. The Digi-Rangers use it's power to become the "Digi-Rangers Zeo". Giving their Digimon the power to Zeo Digivolve.
Former Digi-Rangers; Jason and Kat give up the chance to be Zeo Digi-Rangers, to help out in another way.
Seraphimon tells the Digi-Rangers the power of the Zeo Diamond isn't strong enough to battle Mervamon, so they are gifted the powers of Turbo.
Ian has his Zeo Powers back, it was said Ian still had the powers but just never used them.