輝石 [kiseki] in Japanese. The normal translation for this word is 'pyroxene', although judging by the description of the item itself, that is not the intended meaning and the word is supposed to be referring to a fantasy material. Thus, it is called 'glimmerite' at this site.
Brave Exvius

Buy: - (sell: ? gil)
Drops In: Lanzelut Estiary, Snow Wolf's Fang, Thunder Peak, Decayed Temple
Use: -
Type: Material, Number: 041
Description: A beautiful stone that glows by itself
Profile: A glittering stone occasionally dropped by monsters. Its size and coloring varies by the monster and region, and among them are stones with beauty to rival gems. They're sold for high prices as jewelry, but at the same time there is a not insignificant amount of people who dislike them due to being derived from monsters. Be careful about giving one as a gift.
Category: Item