A few months ago I found this "birthday stones for all 366 days of the year" chart, and both Zoisite (Tanzanite) and Nephrite were December stones.
If I forgot someone else who has a December birthday it would totally suck and ruin this drawing, LOL, but for now Zoi is awkwardly third wheeling and it's funny.
Speaking of the Senshi/Shitennou pairs. I was kind of curious why those stones in particular, right. Kunzite and zoisite are so unusual that I think only gemstone aficionados know them, and even though jadeite and nephrite are somewhat more commonly known names IIRC it still made people mess up the mineral names back in the day.
Well, uh, so, it was alleged that Takeuchi-sensei chose them to match the character and personalities of the senshi they're partnered with, so... Excuse me while I go overthink this and have a meltdown in suffering.
In the first place, supposedly the "inner senshi" are officially meant to be called "guardians"/"guardian deities" (守護 or 守護神). Haven't seen the raws, so I can't confirm, but that's what the JP media says. The term "Shitennou", as many are already aware of, are originally a term used to refer to a set of 4 Buddhist guardian deities.
If true then it just feels like they're all intended to be perfectly matching sets from the start, and it just never made it into the story.
I read that Sailor Moon was originally meant to just end with the Dark Kingdom plot, so maybe sensei intended to have a Romeo and Juliet sad ending where everyone just dies in the end?
These are all based on Japanese gemology guides (same as what I used to research for the Shitennou knight titles), since I imagine it's more likely to be the one Takeuchi-sensei had access to. These may differ from international ones.
(Warning: Long post)
In the manga sensei specified that the zoisite is the blue one, that's also known as "tanzanite". The blue variety of zoisite, among other things, means "calm and intelligence".
The connection to Ami/Mercury's character is very straightforward and obvious.
In terms of romance, a zoisite also means "subdued expressions of love". It really makes me think of how gentle this pair looks in the splash art sensei drew.
Kunzite means "premonition of a lover". It's specifically oh dang when I saw this because Minako was "special" among the senshi in the way that her character vibes are based on "love" (because Venus, goddess of love), rather than the original elemental name attached to the planet. It fits enough to make you pause and wonder if this it truly a coincidence.
The planet venus originally has the element of "gold/metal". The gold is reflected in her design somewhat, but her senshi identity is more related to "love". The other girls' names matches their planets' element names. E.g Jupiter = mokusei = wood, Ki(no) = wood/tree. Mercury = suisei = water, Mizu(no) = water.
Also Venus's relationship with Kunzite ended up being the one who got the most hints actually shown in the manga, so... I do wonder.
The two jades are less overly specific like this, especially Nephrite, but well.
Jadeite is the native jade from Japan, and has been long used to make magatama since ancient times. As such it's always seen as a deeply spiritual stone, that can protect against and/or ward off evil. A perfect match for Rei, who is a miko. One wonders if sensei chose the stone because of Rei being a miko, or if she made Rei a miko after choosing this name for the one matched with her.
Edit: Japanese Wiki claims Rei being a miko was based on Takeuchi-sensei's personal experience as a part time miko. So then Jadeite had to have been named/chosen to fit her, if this theory is true.
Not to mention that Akuryou Taisan, the evil-banishing spell, is something that she uses quite often.
Nephrite is the most confusing one here, because I'm not really able to find the connection. That's why I initially had strong doubts about the rumour/claim about the gemstone names' connection to the senshi.
In the gemology guide that I referenced, its meaning is "charm of maturity". I suppose Mako has always felt like she's older than the other girls, because her hobbies (cooking and home decor) feels really motherly.
Other than that and the stone being a deep green colour similar to Jupiter's senshi outfit I'm not entirely sure. (in the manga, sensei says nephrites are "always deep green" even though IRL that's not the case)
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Just my additional comments after reading everything that I did:
In terms of colour, I like to think Jadeite is a red jade. Not just to match Rei, but in the manga Jadeite's stone is always coloured the darkest (it's almost all-black). Of all the various jades, red is one of the darker-toned ones.
The shading can be arbitrary, especially if sensei was just tracing from reference photos. Dark green jadeite also exist, so for all I know this is intended to be green too.
But if I use the logic that the stones are matched with the girls' character, I think red just fits the best. In gemstone language the red jade means "tenacity" and "willpower". Green jades tends to be associated with things like harmony, prosperity, luck, and so on. It doesn't feel very Rei-like.
Also, the nephrite jade being a "soft jade", it's associated with very cuddly meanings such as "warm" or "gentle", which makes me laugh even more than the "wisdom" one.
Unless, again, these words are more attached to the matching girls' character, rather than the guy in question, because yes I think Mako is definitely very warm and gentle and fluffy and very huggable.
Although other-coloured zoisite varieties were said to have different meaning, it looks like blue is the predominant zoisite colour in Japan. A lot of Japanese gem sites tend to automatically just say "it's blue" while describing zoisite, even though there's obviously other colours out there.
Sensei did specify it's "blue zoisite, a.k.a tanzanite" in the manga, so I think she's aware of the other colours having different meanings attached. Some articles also had mentioned Takeuchi-sensei's family runs a jewellery store?
I wonder if this is why Crystal went with the gemstone's healing properties for Zoi's title though, because the gemstone words overlaps too much with Mercury in a very in-the-face way. The other meanings are Hope, Mystery, Pride, and Noble, and maybe those don't make very good title names? Like, the only viable one is probably Hope, though I feel like it would be even more confusing if Crystal said Zoi is the "Knight of Hope" or whatever.
Another thing about Kunzite is that its meaning also include "pure and everlasting love", but the website caveated this that this is a saintly kind of love (for the gem's association with Holy Mother Mary). So it's not necessarily something to get overly excited about in terms of SenShi headcanoning, but, well, it's there.
"Shitennou" in Japanese pop culture is generally used to just mean "4 very important people", so it didn't have to actually mean anything. Like, a few samurai in history had their "shitennou", who are just the best 4 generals serving under them. Most famous being the Tokugawa shitennou, but other samurai lords actually have their own so-called shitennou. It's just that if the senshi are officially named "Guardians", then this changes the context.
