Rhodonite

Rhodonite is a Gem grown on earth aproximately 5800 years ago. She turned out to be an amalgamated gem (not a fusion!) having one solid gem of partially a black pearl as well as a ruby. She was able to form a normal body, but rather small in appearance. In need of an army and not willing to miss out on having her own warriors she was still accepted by Pink Diamond as one of her warriors and accepted to come to homeworld, yet staying a secret as white diamond may would shatter her.

During her training on homeworld she had to endure the weird stares and derogatory comments of the gems around her. Many of her fellow Warriors refused to cooperation with her, because they saw her an a weak gem, being the only known of her kind and therefor even unable to fuse by common means.
She managed to pass the training anyways and, to most other gems surprise, was one of the bests in her class. 
Pink Diamond must have seen something in her, since she personally made her one of her private soldiers. Or perhaps she meant to rebel against common means further, as to oppose her sisters.

When it came to the war on earth she was sent there to fight against the crystal gem army. She faught many fights successfully for her diamond until the day Pink Diamond was shattered.
Rhodonite was heartbroken. Her diamond, the only gem that saw potential in her was gone. Under new command she still went into battle, though not knowing what to fight for anymore.
As the battle went on she ended in an encounter that left her crystal cracked. Weakend she took cover. The fight seemed lost, homeworld retreated from the battlefield and send an distructive blow from their ships towards the battlefield where many gems including some remaining homeworld gems that where still fighting, but they seemed expendable for the cause. Rhodonite was left behind badly wounded. She lost everything that day, her diamond, her reason to fight and her only reason to return to homeworld.

All of a sudden two other gems stood infront of her. She quickly identified them as a Rose Quartz, the leader of the crystal gems, and a Pearl. Seeing her miserable position Rhodonite told them to give her a quick death and just shatter her, right here and now, homeworld wouldn't miss a dissapointing amalgamation like her anyway.
Rose Quartz seemed shoked upon hearing this statement. She explained that she had no intend in shattering any gem and offered her a save place and healing.
Rhodonite was really confused, by all she knew Rose Quartz was the gem that shattered her Diamond, she expected anything, but not a kind offer of help. Rhodonite broke into tears. Unable to form sentences for what she felt. With little other opportunity she accepted Roses offer.

After Rhodonite was fully reformed and had a chance to explain her story and getting to know the story of the Crystal Gems she decided to stay on earth and help with the protection.
The death of her Diamond was still hard on her so Rhodonite didn't want to stay at the sanctuary. She settled on another part of the earth instead and setup another base promising to keep in contact, if needed. She actually lost connection to the crystal gems for a good few years until she heared the news of Rose Quartz giving up her physical form.
She hasn't personally meet any of the Crystal Gems living at the sanctuary in over 100 years, she felt ashamed that she left them all this years ago. Wanting to know Rose Quartzes son she decided to step out of her comfort zone and set off to pay the sanctuary a long due visit.

Rhodonite is not aware of the fusion called Rhodonite (Ruby+Pearl fusion) back on homeworld.