Now that you've met them all individually, I'd like to talk abt their species as a whole.
Crystallians are made of crystal (or other similar minerals). Their entire bodies are made up of their crystal (think of Land of the Lustrous) but they reproduce organically, much like humans. Common features are face jewels, the number of which indicates a Crystallian's caste, and white pupils shaped like their gem cut. As indicated with the royal siblings, Crystallians can be made of different crystals, and still be related, even identical, even with their unique features.
The face jewel number ascends with rank:
6-Royal
5-Noble
4-Gentry
3-Commoner
2-Peasant
1-Criminal
Being made of crystal, Crystallians' bodies can fracture, and they can even have their limbs broken off, or even their entire bodies shattered; as long as their core jewel is intact they can regrow anything, shard by shard.
While not attached to their body, every Crystallian has an id jewel; it matches the shape of their face jewels, but is larger in size. These jewels allow Crystallians to use their magic, and helps them and others know who they are. Crystallians wear these jewels openly. Every Crystallian can use general magic, and has a special magical ability that many others don't have.
Additionally, Crystallians of the same crystal type rarely have more than that in common. Two Diamonds could have different jewel shapes, colors, and number of face jewels.
Unique to the royal siblings is that they're directly tied with the Crown Jewels, twelve powerful magic jewels that sustain and protect the Crystallian race. This gives them the power to control the elements their related jewel has power over. They can also use the Crown Jewels to boost their individual powers and when casting spells.
Garnet-energy
Amethyst-darkness
Aquamarine-water
Diamond-crystals
Emerald-plants
Pearl-light
Ruby-fire
Peridot-earth
Sapphire-ice
Opal-mind
Topaz-air
Tanzanite-lightning
When used for magic, the Crown Jewela can let the user control their element, or be used like batteries. One jewel boosts power by one, but two boost power by four. All 12 boost power 144x, which can lead to some powerful and devastating spells.
Lastly, Crystallians are kept alive through the magic of the Crown Jewels; if any of them were to break, the entire species would die out.
Less bleak fact: the siblings are dodectuplets, with Garnet and Tanzanite only being a few minutes apart.
