Purple Tanzanite is my sisters OC, but I drew it and came up with a backstory for her since she's not as obsessed with SU as I am.

Yellow onyx we came up with together, her hair style is inspired by red action from ok k.o.! Let's be heroes! And her tail coat is inspired by 1.) human bill cipher and 2.) I like tail coats.

Purple Tanzanite:

She was made for White Diamond originally, but something went...wrong during her creation in the kindergarten.

She was created with an extra set of arms. She was labeled as an off-colour and placed with the others to be shattered. She ran away. She hid with the other off colours, Fluorite, Rhodonite, the rutile twins, and Padparadscha. She was there before Lars arrived.

Tanzanite is carefree, adventurous and fun loving. Tanzanites based on research, are known to be explorers and have psychic abilities. So she can see into people's futures and know what people think and/or feel.

Tanzanites gem is places on the centre of her neck.

Yellow Onyx:

Onyx are known to be serious and logical thinkers.
Yellow Onyx is like a scientist, she develops new gem technology and makes scientific discoveries.
Her job is to work on advancing home worlds weapons and technology and to report to her diamond if she makes a new discovery that can benefit home worlds society.
Yellow is all business. Straight to the point. She is persistent and wastes no time, and she never believes anything without some form of evidence.

Her gem is placed where her eye is supposed to be.

Her diamond is yellow.

She is extremely intelligent and can act a bit condescending at times. 

Yellow and Tanzanites relationship:

Yellow Onyx is secretly friends with Purple Tanzanite.
They are polar opposites.
Yellow is reserved, purple is loud.
Yellow is intelligent and a rational thinker who will stop to wonder if what she is doing is the right thing or will have the best possible result. Tanzanite acts on impulse and does not hesitate, being that she always knows what's going to happen a head of time.
Tanzanite is laid back, Onyx is more uptight.
Yellow uses big words that Purple doesn't always know the meaning of.
They are complete opposites.
But opposites attract, am I right?
They balance each other out perfectly. 

How their romantic relationship began:

Yellow wasn't supposed to be near Purple since purple is an off-colour. People can't ever know about their relationship since purple is supposed to be shattered. She must be careful not to lead anyone to the off colours location.

Purple often questions why yellow likes her or wants to be near her being that onyx is perfect according to the diamonds standards. She's obedient, she's efficient, she came out of her kindergarten perfectly. Whiles purple is "defective". She came out wrong. And she's not like the other Tanzanites, she doesn't function the way a tanzanite should. She's not good enough to be near some one like Onyx.
But Onyx keeps on insisting that Purple is one of a kind and is perfect the way she is. She constantly reassures purple. Eventually after always talking about how great purple is to cheer her up, she started to see purple in a new light and developed feelings for her off-coloured friend.
Purple always loved Onyx though. The whole time actually.
They eventually started dating.

Why they left home world:

One day some guards discovered the location of the off-colours and they fled to a different location. Yellow and Purple got separated from the others whiles running away. They stumbled upon an abandoned and broken down home world ship. Yellow being the genius gem that she is fixed the ship and then they were off to earth where they heard the crystal gems live safe from home world.

After spending hundreds of years on earth they developed and earthy style. Purple Tanzanite wears a sleeveless white sweat shirt and purple tights. Yellow stuck to a style similar to hers on home world, a yellow tailcoat and yellow pants. It was harder for her to adapt since she was so loved on home world. It was easy for purple who pretty much lived in exile where she was at risk of being shattered. Yellow even thought about returning to home world without purple a few times. But she knew that that would be wrong. Plus, who's to say that yellow diamond would even wasn't her back after running away?

Together they fuse to make tourmaline.

GEMS:

Actual yellow onyx gemstone

Actual purple tanzanite

Actual pink tourmaline

Which is associated with healing and loving