Petra's heritage links her back to Denmark, but she was born in America. She lived peacefully with her family until soon after her thirteenth birthday, when a deadly rockslide killed all of her family, leaving her the sole survivor. She never understood why she didn't perish along with her family. She was placed in a foster home, where her foster father acted strangely towards her. While walking through New York City's Central Park, her foster father attempted to molest her. She escaped by accidentally manipulating the earth and grounding him to the ground. This event led her to the discovery of her mutant abilities and the understanding of why she survived the rockslide.
In order to avoid more foster homes and possible events such as this, she created temporary caves for her home and occasionally stole to survive. At 16, she discovered more about her powers - she could turn coal into diamonds. To earn money from this ability, she found a pawnbroker to whom she could sell the diamonds. One day, the pawnbroker made an excuse to call the shop owner, causing Petra to flee. She woke up in a holding cell, where she was told she had a visitor. This visitor had dropped all charges and convinced the authorities to release her into her care. This visitor was Moira MacTaggert.
She eventually joined the X-Men team known in the Deadly Genesis crossover and was killed during the mission against Krakoa.
Her powers/entity live on in Vulcan. Darwin transformed Sway, Petra and himself into energy that was absorbed into Vulcan. Darwin was later extracted from Vulcan.
Petra has the ability to manipulate earth and rock. This power allows her to perform various feats including (but not limited to) turning coal into diamond, creating (or stopping) earthquakes and rockslides, and making the earth swallow up or trap enemies.
In Wolverine and The X-Men, a young girl appears as the daughter of Maverick. This girl resembles Petra in appearance as well as powers (geokinesis). Wolverine is sent on a mission to kill her father but when he doesn't, Weapon X sends Sabertooth. Years later, Wolverine comes looking for answers and Petra attacks him, thinking he killed Maverick. Later, she finds out Sabertooth was the attacker and telekinetically moved a tree to push him off of a mountain. This could be another character all together, considering the greatly different backstory.
What if the Second team of X-Men had lived?
In this reality, the team comprised of Vulcan, Darwin, Sway and Petra had not died during the battle against Krakoa. The original X-Men died. The team became the X-Men. They were able to finally beat the Brotherhood and Vulcan is viewed as the greatest hero ever. It was revealed that Vulcan was responsible for the deaths of the original X-Men. He was sent to live on the island and watch the event over and over for the rest of his life.