It appears that the author of this work invented the word 雪華 (sekka); it originates in that book. Other than that, a snow crystal is just called ゆきの結晶 (yukinokesshō).

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gsf_emergency_6 4 hours ago | parent [-]

Related(?) word meaning sublimation

https://ja.wikipedia.org/wiki/%E5%87%9D%E8%8F%AF

1828 (4 years before this book launched)

https://bunka.nii.ac.jp/heritages/detail/517539

(Sword guard)

	▲	kazinator 2 hours ago \| parent [-]
		The word doesn't appear on the sword guard itself, though, so the object doesn't attest it.