Steven Universe is full of mysteries when it comes to the Crystal Gems' backstory, and Pearl is the most enigmatic of all the characters. She's a fascinating person: courageous, neurotic, and compassionate in equal measures. Slowly, we've learned more about her, and she has one of the most interesting origins in the story.
Just A Pearl
Steven Universe season 2 has gone a long way to filling in the blanks about Homeworld, and the details of the gem hierarchy. We've got more information about the Diamond Authority that Ronaldo warned us about, with Blue and Yellow Diamond finally making a brief appearance. It's Peridot's arrival that has prompted a lot of this information to come to light, as she explained a lot of Homeworld's society and plan for Earth to Steven.
And it's Peridot who provided us with some vital details about Pearl's place in gem society.
"She's a made to order servant, just like all the other hundreds of Pearls being flaunted around back on Homeworld."
This revelation is huge, and explains a lot of Pearl's low self worth issues. Earlier on in the show, she had referred to herself as "just a pearl", and now we know she is essentially considered property on Homeworld this adds a note of tragedy to that statement. But Pearl's grown a lot in the last few episodes, with a lot of help from Garnet and the others. Which means that when Peridot started treating her the same way she'd been treated on Homeworld, her response was somewhat more self confident...
In a lot of ways, it doesn't matter who Pearl originally belonged to: it's her journey to become a hero that is one of the most poignant plotlines in the show. But knowing how she defected, and what prompted her to join Rose Quartz's rebellion, makes for a compelling story. So here are the main possibilities for Pearl's origins...
She Belonged To Rose Quartz
This is definitely possible, and would explain why Pearl sided with Rose: she was already Rose's servant. It would also explain her dedication and loyalty to Rose. But this would erase some of her free choice, so personally I'm hoping Pearl didn't initially belong to Rose. That way, her defiance of Homeworld is entirely her decision, and her love for Rose isn't clouded by complicated issues of ownership.
There's actually quite a bit of evidence against Pearl belonging to Rose, and it comes down to these two points: Pearl's gem placement, and her skin tone. In the other Pearls we've seen (Blue Diamond's Pearl and Yellow Diamond's), their gems were located in the same place as their mistresses'. But while Rose Quartz's gem is on her stomach, Pearl's is on her head. And that might be our strongest clue as to who Pearl's owner really was.
Pearl Belonged To White Diamond
In the episode It Could Have Been Great, the Crystal Gems travel to the moon to find out more information about the Cluster. On the moon base's walls we see a giant mural of White Diamond. Note her gem placement...
It's on her head, just like Pearl's. And this isn't the only clue: Pearl's skin tone is also very pale. Although Pearl's hair is salmon pink, it's possible she changed it (like Garnet did after her pink and blue style). In the episode The Answer, all the gems are depicted primarily using the colour of the gem they belong to. And Pearl appears to be far whiter than usual.
(Hat tip to Cartoon Universe on YouTube for pointing this out.)
This is pretty much all the evidence we have so far, but my money's on Pearl originally belonging to White Diamond. There is actually a third option: as Pearl wears yellow, blue, pink, and white in her clothes, this has lead some fans to believe she belonged to all the diamonds... which could be interesting! Hopefully we'll get the answer to this soon, as a flashback episode to how Pearl joined Rose Quartz would be awesome.