I saw a couple of things of people having the idea of the Celestial Stones being living beings of some sort, so I thought I'd give it a try too, with my own headcanons and fake study on them! I also threw in the Starshard because I wanted to expand on the idea around it.
Celestial Stones:
One thing is for sure, is that these stones are not from earth, nor is their magic, which is highly powerful and extremely dangerous, especially in the wrong hands. However, not much is known about these strange creatures, their homeworld or even their history. But there are many theories and studies on them. There's also the idea that there might be more than the two, whether their other Moonstones and Sundrops, or different kinds of stones, but no one has seen another, or where they come from. Some say the heavens, and some say another plant.
These creatures have a strange way of communicating, in that they don't actually speak with words. Instead, they talk through thoughts, feelings, emotions, memories and dreams. They can speak like a human, but it appears to be difficult, or they use the images of a person to speak through (As evident by Rapunzel's dream of one of them talking through a vision of Varian). They can recite their incantations, or pick up on certain words to speak with, like names. For a human, this way of communicating can feel strange or confusing to understand, often missing the message behind what a stone is saying.
Celestial Stones appear to be somewhat shapeshifters, able to craft a body of their design around their core. They are also genderless, their voices and sounds varied, and often come across as simply alien-like.
Demanitus himself theorises that there might be a war going on where the Celestial Stones come from, as evident by the Sundrop and Moonstone crashing to earth, parted from one another and weakened when they first landed.
Sundrop:
Being the more friendly one of the two stones, the Sundrop is a gentle being around humans, having been not too picky when it came to a host. However, it has been noted that it does behave a little condescending towards people, treated humans like exotic animals or children, no matter their age. Though it's never malicious about it, the Sundrop just tends to view people like that, as a result of being a powerful entity. But it is willing to help out if a human needs it, but only if it deems the situation too troublesome for an average mortal to handle.
The Sundrop has no sense of personal space, often picking up objects, animals or people, examining them with excitement, but tends to put them right back when it's done, being gentle a possible. If it grabs you, the best you can do is stay still and want until their done.
They also rarely attack, mostly using defensive tactics in a fight, unless pushed far enough. It takes a lot to get the Sundrop angry, but you better run and hide if you somehow do piss it off. It can construct and burst magical rays of blinding light, able to break down almost anything it hits, even creating a nova of energy when distressed. And, of course, it has the ability to heal and bring almost anything back to life, something many craved to have as their tool.
Moonstone:
They are very much the opposite of the Sundrop in many ways. Cold, quiet, harsh, not particularly fond of humans, especially the ones that try and touch it. There have been a couple of people that peaked its interesting, but never enough to make them their host, wanting to be alone in its search for their lost light. They don't like to get involved in human problems, leaving them to deal with it on their own, as it takes a lot to convince the Moonstone to help in any way. That being said, it doesn't actively go out and attack humans, only when it is disturbed or one is in their way.
The Moonstone actually hates Cassandra, as the only reason she could become its host was because she caught it off guard. The stone was opening itself up to bond with the Sundrop again, which is also the same way they merge with a human, so Cass took in at the right moment. However, the Moonstone did not accept her, meaning they didn't fuse entirely, being the reason why it wa mostly had control over it, there were small moments where the Moonstone rebelled, like making a simple slip up back in the fight in the black rock tower, lowering its defences so it could be cracked, not wanting to harm its Sundrop.
When the Moonstone does attack, it uses its black rocks, forming them from the ground or off its body. The rocks allow it to spread its range of magic, able to perform the decay incantation through them or changing them into red rocks. Its also how it can scout and explore, able to sense through them.
Sundrop/Moonstone:
It is well known that the two Celestials Stones are connected, having a desire to be united again. Their relationship is described to be very loyal and deep, some even saying it might be romantic in nature. Demanitus theorises that, if there are other stones, that the need to pair up could be part of their biology. It is unknown whether their connects are made at birth, or a bond needs to be formed to create one.
When the two were separated, the Moonstone began to search right away, being the more open one in their protective side over their lost half. The Sundrop decided to try and use a human host to take them back, as their powers had no way of making their way to the Moonstone on their own. The two were desperate to be together once more, and would combine their powers to be able to be sent back to their homeworld.
The two can fuse or separate at any time when close enough, able to form into an even stronger being when needed. When they reunited after Zhan Tiri's defeat, whatever damages they received from falling to earth were wiped away, now returning to their true forms.
Starshard:
The odd one out, being the first artificial Celestial Stone ever created, by a human no less. With strands of magic from both the Sundrop and Moonstone, crafted together by science and other magics, the Starshard is rather strange, even by alien standards. To put it simply, it didn't understand what it was, only having bits and bits of traits from its doners, not knowing what it was supposed to do or who it is. This is one of the reasons why it refused to work for Demanitus and other humans for a long while.
Then it found a host in Varian, but it was still behaving strangely for a Celestial Stone. When the alchemist finally learnt of its existence in him, the Starshard began reaching out to him, seeming a lot more eager to know its host than the previous stones. That's because the Starshard has the trait of desiring a connection, a counterpart, but doesn't know what it is. So it has decided that Varian is its other half, seeing him more than a host, which is why it talks to him often, doing everything it can to support its human.
Its physical form reflects its nature, coming across as an unfinished mannequin of sorts. The Starshard mimics Varian and the people around him very often, picking up on human tendencies in an efforts to develop itself, to learn about the world it lives in. So its form is bland and unfinished, wanting to change it over time the more it sees of earth and humans. It did, however, copy Varian's hair and body shape, being the person it spends the most time around. It will also shape up clothing from time to time, often matching what everyone around it is doing. E.g. if everyone is getting ready for a fancy party, it will make and outfit to fit the occasion based on what everyone else is wearing. Unlike the Sundrop and the Moonstone, The Starshard loves humans and wants to learn all it can about them, having never been to the Celestial Stone homeworld, knowing only earth where it was born.
