The name Luminium is derived from the English root lumin-, which was in turn derived from the Latin word lumen, which means light and brightness.
Luminium is a masculine variation of the original gender, Aetherium. All genders in this family describe not just a connection to heavenly bodies and the otherworldly as gender, but as feminine, masculine, and androgynous presentations of that gender.
Before being officialized with a name or flag, Aetherium was simply referred to as Calliegender, as a sort of running joke to play on the indescribability of the creators gender identity. It's first proper dedicated conceptualization was in the form of a starter pack image. Several months later, the creator Calliope made and published the official gender and flag, first in its Discord server and then immediately after on their Tiktok.
After a large initial response to the gender, a follow-up video was made with its masculine counterpart Luminium, and later on a third video for the androgynous/combination variant, Alstrium.