What is the necklace that Hina is wearing? In Weathering with You, Hina is wearing a blue crystal thing necklace.

 

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What is the necklace that she's wearing? Is it just a decoration or...


   a talisman of some sort?
I don't know if it's official or not.

They're called "Clavis". 
Following to this movie called "Children Who Chased Lost Voice" or as known "Hoshi wo Ou Kodomo", in some scene as refer:


   giving Asuna his clavis, a crystal, and fights back. The Arch Angels interfered, annihilating the gate keeper and the Arch Angel commander captures Asuna and uses the clavis to open a gateway to Agartha.
 


So they're called Clavis crystal / stone. 
The main power itself may vary in different characters and the movie itself, as we known like : 


   
 Ryuuji used clavis to attract god of death, when to bring alive her wife, Lisa and also a key that allows entrance into Agartha, the actual underworld featuring the gate between life and death (in Hoshi wo Ou Kodomo / Children Who Chase Stars)
 Mitsuha and Taki switched their bodies in over time (in Kimi no Nawa / Your Name)
 Hina controlled the weather by praying to the god (in Tenki no Ko / Weathering with You)
 


References: link1, link2.