I think it depends on which franchise you look at. both the manga, 90sAnime, and PGSM have different functions for the Silver Crystal
(no matter how ill-defined they are at times. <<; ).
90s Anime wise: I think it's a tool. It's got no sentience to itself and it can easily be passed down to another. We also don't have that idea that when past/present Silver Crystals are close it could cause big problems. Chibiusa doesn't seem to gain her -own- Silver crystal but rather the original one bonds/goes into her making her the new owner.
And consequently, making Crystal Tokyo defenseless when she goes into the past to stay. But that's a different discussion.The thing with the rainbow crystal in-universe is because they were Big Baddies and sealed into the Silver crystal during the fight against the Dark Kingdom (I'm recalling this from memory <<;; I could be wrong but it feels about right? ) So, it doesn't seem like they influenced it much one way or another in how it's used in this version.
It seems like the Silver Crystal activates because of Usagi's emotional distress/issue during the first season. It's only as powerful as Usagi and the others invest in it. Seems like it amplifies and uses the power given to it for a specified task.
In PGSM :it's still a mindless tool. Usagi's emotions (her heart) controls it but it gives out power every time she uses it or is upset. It's treated as a double edged sword in that it's useful to use against the enemies but because of the uncontrollable power, it makes their worst enemy
stronger.
Manga wise: This is where the question of whether it's sentient or not comes in more. I personally think it's either half sentient or has an advanced AI. Which would make sense, given that it doesn't respond directly to just anyone but those of a specific lineage (Q. Serenity, Usagi, and Chibiusa respectively.).
I still wanna say it's merely a tool in some regard because it needs direction, otherwise it's just kinda there. Hologram!Q.Serenity states (and this is overall in the franchise) that the Silver Crystal follows Usagi's heart. Hence why a piece of it went into Mamoru when he was more or less dying before she awakened as a princess. She was desperate to save him and it did what she wanted, though not necessarily
how she wanted it as I doubted she wanted the crystal to slice a piece of itself off and go into him.
Then you have to figure in how Queen Serenity, herself, used the Silver Crystal too. She's shown to have used it twice- once for Queen Nehellenia and the Dead Moon and again for Queen Beryl. There's also a difference in how she used them there too. When Q. Nehellenia showed up and tried to hypnotize/drag her darkness in with her, Q. Serenity was quick to try and seal her away. Obviously, while distressed, she was focused and her emotional state wasn't in disarray. She managed to not only take Nehellenia into the crystal itself but then redirects it at her own personal mirror to seal her into it.
- Spoiler:
This implies that the Silver Crystal can also be a temporary containment thing for darkness before you put whatever you want to seal away with the intent on doing so.
The next time we see it used by Q. Serenity is when Sailor Moon and Tuxedo Kamen have been encased in the crystal and are essentially dreaming/reliving what happened in their past lives. We see that Q. Serenity was really in disarray and distraught so much over the deaths of her daughter and everyone else that the seal she made while sending everyone to a non-specified future was incomplete.
- Spoiler:
So, fast forward, it's with Usagi and the way Usagi has used it may have been unique to Usagi (P.Serenity) herself. We, unfortunately, don't know a lot about P. Serenity's childhood overly much or how the Silver Crystal is normally passed down.
I get the feeling that the way it should've been used originally was changed fundamentally with the abrupt end to the Silver Millennium. Unlike what we've seen from the past glimpses we get of Q.Serenity with it, it always is in the Crescent Moon Wand that she summons- for Usagi it's separate from the wand for the most part and it only seems to open in it's lotus form when the proper prayers/energies have been focused into it. Whereas before it's always been in it's diamond closed shape.
The fact that it was inside of Usagi when she was reincarnated may have altered that specific Silver Crystal fundamentally enough that it became a part of her. And may have actually became apart of her original Sailor Crystal.
This may also be why Chibiusa cannot use her mother's Silver Crystal at all in this version. For her, it's just a trinket. It won't give her power and it won't do as she wishes, despite her being Usagi's one and only daughter and heir.
Considering how unique Chibiusa, herself, is and the daughter of two crystal holders like Mamoru and Usagi- it just heightens her eventual creation of her own Silver Crystal. (which I think is modeled after her mother's. Since Endymion's seemed to have been kept either inside of himself or inside of his staff-sword. So it's never readily seen or available for Chibiusa to even think about touching or the like.) And how that crystal eventually became the Pink Crystal.
*scratches head and looks up.* Er..
*coughs.*
tl;dr version: My opinion, the Silver Crystal functions like an AI rather than actual true sentience and does generally what its' holder wants but only in a vague fashion if the holder isn't focused at all on a
very specific thing.
Like in the Manga: When Usagi awakens, her desire is to save Mamoru. During this moment when the crystal is revealed, it heals -everyone- until she's able to direct it's focus
specifically at Mamoru.
It looks like it can only handle one order at a time. (Q. Serenity's half sealing of Metallia and reincarnation of everyone.)
Soo...yeah. that's what I got from it.