The Crystal Temple originally had rooms for Ruby and Sapphire.
Now Garnet doesn’t use them, she most likely lives in the Burning Room. We don’t know when that switch happened, it must have happened whenever Ruby and Sapphire decided to live as a fusion permanently.
But before Ruby and Sapphire decided to permanently fuse, they used to live in their own separate rooms at the Temple, just like Rose (top), Pearl (upper right) and Amethyst (upper left).
Based on that, and the prejudice Homeworld holds against fusions, I believe that Ruby and Sapphire first fused on Earth.
They might have had a close relationship in the Homeworld, which continued and developed stronger while they lived on Earth with Rose, but I think they decided to become Garnet and stay as Garnet only on Earth.
Garnet considers Earth her planet (”Get OFF my planet!” “I won’t let you hurt my planet, I won’t let you hurt my friends.”). To me, this implies that she was first “born” on Earth, many centuries ago.
Pearl’s and Amethyst’s rooms connect, so it’s likely that theirs did too.
Imagine Ruby sneaking into Sapphire’s room to spend more time together outside of missions.
Imagine Sapphire singing in her room for Ruby to hear, when she knows that Ruby is feeling upset.
Imagine Ruby and Sapphire forming Garnet for the first time, as their first fusion.
Imagine Garnet awkwardly trying to work out her limbs, her speech, and excited feelings the first few minutes, like Stevonnie.
Imagine Garnet skipping around the beach laughing, diving, rolling around, testing her new powers on boulders with a grin on her face. (“You are an experience. Now go. Have. Fun!”)
Imagine Ruby and Sapphire defusing after that, the happiest they’ve ever been.
And here is Reed’s kid sister, Ruby! She’s 8 years old, and is very outgoing and active! She loves nothing more than running around in the grass, swimming at the community pool, making new friends, and coloring in her spare time. She loves animals, and daydreams about becoming a Veterinarian when she grows up. When not playing outside, Ruby can be found lounging in her room, which completely reflects her colorful personality!
Ruby lives on the top floor of the bed and breakfast in a modestly sized apartment. Her bedroom at the inn, surprisingly lacks red. She likes to keep her room a calm place, with many shades of blue’s and greens.
Aric’s busy day was, thank the Gods, drawing to a close. He’d suffered through meeting after meeting and though he was supposed to meet Eira in the nursery, all he wanted was a bit of rest. He should be spending every spare moment he could find with baby Theo, but seeing his youngest son now would only tire him all the more. His strength and his will would be better with a bit of rest before he tried to spend time with his family.
He had removed his cloak and placed it over the chair where he typically left it when they slept, stretching his arms as his eyelids drooped over his eyes. This nap was well-deserved; perhaps he wouldn’t wake from it before nightfall. Perhaps he wouldn’t wake from it at all. He turned and stared into the fire when he heard a noise behind him and saw the door pushing open. He assumed it was Eira and he smiled before he set eyes on Ruby. Startled, he took a step towards the door. “Ruby?” he asked, as if he were unsure of her presence. “What are you doing here?”