Fluorite is a power stone ore that emits beautiful sparkle when put in fire, just as as firefly. But in the end of the day it crumbles, just as Hotaru childhood friend in the nurse room.
She only could say "I'm fine" before fainting, so I brought her to the nurse room.

I gave her flowlight as a present in the childhood, and she was very pleased with it. I was not aware of "fluorite" alias back then.
Hotaru is a regular at the nurse room. I peek at her medical card. Lung cancer. Hotaru only has several months to live.

[From vndbreview]