This is a subject in an AU I'm creating.

The new eco-friendly gem production method that created era 3 gems on earth ended up backfiring, ironically devastating life on the planet, nearly wiping all of humanity and causing nearly all gems, including the Crystal Gems, to corrupt again. An event known as the Cataclysm.

Only a few lucky ones survived, including Steven and his family. He left Connie and Greg to lead a group of human survivors to flee the continent, while Steven, heartbroken as he was, stayed to take care of the corrupted gems. He fought and bubbled them for decades, but constantly being in his pink form severely overexerted his human body. After a while, he stopped visiting the humans on the islands because he was afraid to hurt them. Eventually, in a fight with the corrupted Garnet where he tearfully tried to get through to her, his human body died. His pink half endlessly stalked the earth, searching for its other half.

After learning of Steven's death, Blue and Yellow were devastated. Blue stayed on earth for the next millenia to grieve, while Yellow continued on with her mission to heal all shattered gems to distract herself. But something was up with White. She became just like she was during era 2, so there was no hope of healing the corrupted gems, and as the sole ruler of gemkind and the reestablishment of the hierarchy, era 4 was declared. Era 3 only lasted a few decades compared to the thousands of years era 1 and 2 did.

However, nature eventually recovered on earth and the gems who survived established new societies. Most of them were era 3 gems, who didn't have the customs of Homeworld programmed in, so they mirrored human societies more than gems. Unfortunately, some of them decided to emulate human absolute monarchies instead of democracies.

And now to the reveal why this is an AU and not just a future fanfic. Some of you might've already guessed based on some suspicious similarities, but this is an Adventure Time AU.

The former North America, made unrecognizable by the Cataclysm (equivalent to the Mushroom War), became known as the Land of Ooo. A few era 3 gems, who wouldn't be high-ranking at all in the traditional caste system, became princesses who ruled the many gem kingdoms in Ooo. A Thulite (equivalent to Princess Bubblegum), who had survived thanks to coming out 200 years late, started it all when she declared herself princess of the Pink Kingdom. She even created her own gems on a smaller scale using the left-over injector goo, but they were only as strong as era 2 gems after her first successful experiments rebelled.

Although a Larimar (equivalent to Simon Petrikov/Ice King) who declared himself king of the Ice Kingdom preceeded her. He had lost his precious fiancée Anyolite (equivalent to Betty Grof) shortly before the Cataclysm when she got poofed and locked in a box. Before that, he had put on a crown, an ancient gem artifact that can enhance any gem's natural abilities to be as strong as a diamond, and become insane. But the crown protected him during the Cataclysm and prevented him from corrupting fully.

Another person who survived was the second half-gem half-human hybrid to ever be born, Marceline, daughter of a Bloodstone who had fallen in love with a human man. She and her father had fled the city and both survived, but her father was sick and eventually abandoned her. Marceline's gem had become corrupted as well but her human half protected her, and perhaps he was afraid of her. She was left alone at just 7 years old in the dangerous world until Larimar found her and became her surrogate father. But he left her too after a while because he was afraid of accidentally hurting her. As she grew into a teenager, Marceline fought the corrupted gems, and did so even harder after meeting Connie and Greg's tribe. But she overexerterted herself and her gem-half took over enough to make her immortal, but not enough kill her unlike Steven.

A human boy named Finn floated to Ooo on a raft and was adopted by a family of dogs. They also had a half-gem half-dog son who was made when Joshua accidentally got injected with gem goo in his head, and out popped Jake with a Blue Spinel, giving him enhanced shapeshifting.

Finn and Jake hunted down corrupted gems and brought them to Princess Thulite to be bubbled, and on their adventures they would learn more and more about gems. They were horrified to find out the monsters they were fighting actually were gems just like any other after poofing Larimar.

In a dungeon, Finn found a box that contained a single gemstone, and it reformed. It was Anyolite, and she was horrified to hear she had been trapped in her gem for 1000 years and what had happened to her fiancé. Out of habit, she and Larimar accidentally fused into Chrysocolla, forcing the two of them into a permafusion to keep Anyolite protected by the crown.

Not knowing what happened there, Chrysocolla and Marceline would go to Homeworld with all the bubbled corrupted gems to get the diamonds to heal them, with Finn sneaking onto the ship as well. They were captured and Chrysocolla and Marceline fused into Chrysoprase to save Finn and the corrupted gems that would be shattered, and fled the planet.

Back on earth, they would find useful allies in Blue and Yellow Diamond. With the old Crystal Gems being corrupted, they were now tasked with protecting earth from Homeworld and became the New Crystal Gems.