The first time Pearl fused with Pink Diamond was after Garnet joined their ranks. Before that, neither of them had ever considered it, or even known that such a thing was possible. But Pearl knew what was coming - Pink Diamond loved new and novel things, and fusion was almost all she could talk about for weeks after meeting Garnet.
So it was no surprise to her when Pink Diamond finally squealed "Pearl, we should try to fuse!" one day while they were alone together in Pink's palanquin. "Don't you think it's just so fascinating? Two completely different gems becoming one and sharing everything - their ideas and feelings and secrets." She said dreamily.
It certainly did sound interesting. Pearl always wanted to be as close as possible to her Diamond, and how much closer could you get than fusion? It would be a lie if she said she hadn't imagined it herself. But as nice as it sounded, she still had her reservations. "I'm not saying no,my Diamond, but… are you sure that's really a good idea?"
"Why wouldn't it be?" Pink frowned.
"It's just… it's forbidden. Especially for a Diamond. I can't even imagine… other gems would find out…" Pearl was already thinking about what would become of her if they were found out. Pink would be fine, but she was sure her own gem would be swiftly crushed into powder and scattered to the wind. And if the other Crystal Gems saw, that could be equally catastrophic for different reasons.
"So we'll just go somewhere private. No one has to know. There's so many places to hide on Earth." Pink reassured. "Besides, I wouldn't let allow anything to happen to you, even if we were discovered." But Pearl wasn't swayed yet.
"And...and...it would be improper. A Diamond fusing with a Pearl? Fusion between different gems is already unheard of, but that's even more unheard of!" Pearl stammered.
Pink looked hurt, and Pearl instantly regretted what she said. "Like I would care about something like that. You should know that doesn't matter to me."
"Of course. It's just that I was worried about you, well…" Pearl was going to say she was worried about Diamond lowering herself to fuse with someone like Pearl, but she realized that wouldn't make her Diamond happy either. Plus, that was just Homeworld's way of thinking, not Pink's. "I'm sorry. I wasn't trying to imply that you would be upset about that. It's just hard to overcome something you've been conditioned to think all your life."
Pink leaned down and gave her a hug, which Pearl eagerly returned. "Don't worry, I understand. It's not your fault." She smiled and looked down at Pearl. "And it's totally okay! If you're nervous about it or you don't want to fuse with me, you don't have to. I just wanted to see what it felt like, but it's not that important. Maybe one day another opportunity to fuse will come up with a Crystal Gem or something, so it's not a problem."
"W-what? Fusing with another gem?" In that moment, Pearl thought she understood what it felt like to be shattered. Pink was trying to be kind, but the idea of another gem fusing with her Diamond - when she had wanted to fuse with Pearl - was almost enough to make her cry. "No!" Pink's eyes widened, startled by the sudden outburst, but Pearl continued. "I mean, that won't be necessary. I'll fuse with you! I trust you."
"Really?" The expression of pure joy on Pink's face almost made Pearl's knees go weak. She made sure to stash it away in a private, happy part of her mind that was reserved for every time Pink smiled at her. "Thank you so much Pearl!"
That night, they moved the palanquin to a part of the Earth that had few humans and no gems. They were deep inside a forest so dense it was almost hard to see the sky, except for a few spots where the moon was able to shine through the trees.
Pink and Pearl were both a little nervous. Pink was acting less enthusiastic and kept shooting over shy glances as Pearl tried to find the perfect spot where they could move around a little without being hampered by the trees. Pink was so big it was a little hard to navigate.
"All right, this will do." Pearl said and stopped to look up at Pink with a blush.
"Are you sure you're okay with this?" Pink asked her and Pearl nodded.
"So uh, how do we fuse?" Pearl asked.
"Right! I asked Garnet before -"
"Oh... you told Garnet?" Pearl asked, her voice slightly panicked.
"No no, don't worry, I wouldn't tell anyone! I mean, I asked her because I was curious." Pink reassured quickly. "She said the first time it was an accident and Ruby and Sapphire just kind of fell into each other. The second time, they were dancing when it happened."
"Hmm." Pearl wondered which would be best for them. There was quite a size difference between them - while Pink was smaller than the other Diamonds, Pearl didn't even come up to Pink's gem. "Which do you want to try first?"
"Dancing sounds like more fun, I think! I want to try that. Oh, but wait! Here, I have an idea." There was a flash of light as Pink shapeshifted - shrinking down and emerging in the form of a Rose Quartz. It was the form she had been using to lead the Crystal Gems for a while now, so Pearl wasn't surprised. Though she still towered over Pearl, it would definitely be easier to dance with her.
Pink made the first move, reaching out and taking Pearl's hand and pulling her in closer. Pearl shyly put her other hand on Pink's waist. "Is this okay?" She asked, and Pink nodded.
They started to sway and move together, and pretty soon they were dancing their way through the forest clearing. But... fusion wasn't coming, even after they had been dancing for quite a while.
"How long is this supposed to take?" Pearl asked nervously, worried she was doing something wrong.
Pink just shrugged. "Your guess is as good as mine."
Their whole dance was a little awkward. Pearl was having trouble relaxing and getting into it. Pink's form as Rose made it a bit easier to forget that she was attempting to fuse with a Diamond - which was a lot of pressure in itself - but thoughts of being found out and shattered were still playing over and over in the back of her mind. She wasn't really worried for herself, but being separated from Pink was a worse punishment than the shattering part.
"Calm down, you're so tense!" Pink laughed and playfully dipped her. "You're thinking too much again, aren't you, my Pearl?" She brought her back up and rested her forehead against Pearl's gem. "Just relax and have fun."
It was the first time Pink had ever used 'my Pearl' as an endearment, and it brought tears to Pearl's eyes. For Pink to show her that level of respect and love...she almost couldn't believe it, and she could feel any last reservations she had leaving her mind. It was silly to worry about anything else but Pink. She wasn't afraid anymore, all that mattered was getting closer to her.
And then it happened. Fusion was an odd thing. It almost felt like shapeshifting, but also like melding into someone else's body. It was fascinating and terrifying at the same time and Pearl felt like she was losing herself and transforming into someone else.
She opened her eyes and she didn't know who she was. The world was a confusing and new place to her and she didn't understand anything. The thoughts in her mind were all a jumbled mess. One voice in her mind was large and effortlessly loud - in comparison, the other was much smaller and was being drowned out, even though it was exerting more effort to be heard.
It felt like she had been weak all her life, powerless and frail and frightened and always at the mercy of others. But now she was strong. Unimaginably so. Maybe even more so than a Diamond. Something in her was desperately yelling 'a pearl was never meant to have this much power,' but she wasn't a pearl, so she only entertained the thought for a moment and then ignored it. She was more curious to learn about the world around her and about herself.
As she explored, she became to feel calmer and more at ease in her own body - the larger presence in her mind was somehow soothing and lowered its voice to whisper reassurances, and the other presence seemed comforted. She felt loved by both of them. How could she not? They were both herself and she was both of them, and they were so happy she existed. She couldn't explain it - but then, she couldn't explain much about herself.
The forest was very interesting. There were some little creatures that glowed and lit the way in front of her. Some other creatures stalked through the trees. She was scared of them, but then realized 'oh, they can't hurt me anyway.' Nothing could hurt her, ever. So instead of being frightened, she contented herself watching them for a while.
She explored in the woods until the sky started to get brighter. For some reason she felt exhausted, even though she hadn't done much. It was like just existing was using up all her energy. So she laid down in a grassy area, stared up and watched the darkness change to light.
Pearl felt herself pop back into her own mind and body. She was on her back, facing the sky. For a moment she just laid there speechless - there was no possible way to describe how she felt. It was like suddenly she had become so powerful that it overwhelmed her entire being. She had screamed as loud as she could so she could be heard and wouldn't be lost in that power - but strangely, the power was also hers to wield. Pink was so much stronger than her, but she had somehow managed to reign it in and split it between them evenly so that Pearl's mind wouldn't be crushed under the weight of a Diamond.
More importantly than that, she had felt so very loved. She couldn't tell where she ended and Pink began, but the whole time it felt like she was being enveloped in love. Believing Pink loved her was completely different than experiencing it first hand. It was the most incredible thing she had felt in her life and she almost wanted to fuse again immediately just to get that feeling back. It was addicting. She couldn't help but wonder what Pink had felt from her. Normally she would be embarrassed by a thought like that, but she was feeling too good to worry at the moment.
Pearl rolled over in the grass. Pink was already watching her with a fond expression, and Pearl blushed a little - she had been so caught up in what she was feeling that she hadn't even noticed.
"Wow." Was all Pink said, and that was all that needed to be said. Pearl shared the sentiment completely. Pink reached out and gently pulled Pearl closer to her so she could cuddle with her. Pearl buried her face against her.
"Let's fuse again soon, my Diamond." She murmured.
"You changed your mind about fusion fast, huh?" Pink laughed a little. "But yes, I agree. Let's fuse again soon, my Pearl."
Notes:
I imagine fusing with a Diamond would be pretty difficult for a Pearl to get used to mentally for the first time! I hope I conveyed that well.
I think Rainbow Quartz (or what she was called back then) would be a lot bigger than we saw her in the series, she is a Diamond fusion after all. It would probably take many attempts for them to customize her appearance into something they liked.
Rebecca Sugar said in an interview "no one calls their pearl 'my Pearl.' People only say that to their superiors." So for Pink Diamond to call Pearl that for the first time would be a pretty big deal for Pearl. So that's why I decided to have that be the catalyst for them finally fusing.