The idea came to me while I was binge-watching/reading Houseki no Kuni. This is an OS collection so even though every chapter takes place in the same universe, each chapter can be read as a stand-alone. They also don't happen in a linear timeline.

FLASH BANG

Summary: Or how Phos ate and gave people a heart attack.

"What's that?" Phos asked, hand reaching for the bright yellow cylindrical object Power Loader had left on his desk. They rose it to eye-level to examine it, turning it around. It was small enough it fit in the palm of their hand, with a lever and a ring on its side. A grenade.

"Miniaturized flashbang," Yagi said, confirming what Shouta had guessed. Something however made his stomach twist. "You press the lever," he added when Phos made a confused expression. "Pull off the ring while still pressing the lever, then release and throw it. It'll produce a blinding light and lots of noise. It's for stunning people."

They gently traced the lever with a finger, looking far too curious for Shouta's taste. He didn't know what exactly the gem was thinking but the instinct he had developed around his students and charges told him he wasn't going to like what would happen next.

"Phos, you can't throw a flashbang inside!" He was already on his feet, cursing Yagi for his natural desire to help, Power Loader for leaving a flashbang unattended and because his desk was on the other side of the staff room. Of course, the one he cursed the most was Phos, their unending curiosity and complete lack of sense.

Phos pressed the lever with a thumb, pulled the safety ring off and promptly threw the grenade inside their mouth.

Shouta stopped right in his tracks. He heard the other teachers currently in the room gasp.

Light erupted from Phos' eyes and mouth, along a bang and a series of shattering noises he had come to associate to the gem breaking.

Fortunately for everyone, Phos had a fine enough control of their quirk—it wasn't technically a quirk but it was so much easier to think of it as such—that nobody had to see their head shatter, even though it was still disturbing to watch the veins liquid gold keep them glued together. They were left with a blue face where the powder had flaked away. The same blue speckled with gold as their hair, the same material. What gems really looked like without their powder, Shouta realised.

"Oh my goodness! Phos, are you okay?" Yagi's worried voice stirred Shouta out of his thoughts. They both approached, Yagi, as soft-hearted as always, stepping right in the gem's personal bubble to examine them.

"Care to explain why you did such a stupid thing?" Shouta needed a pay raise.

"I eat light. Toshinori said it made light so I had to taste it." A big one he thought when Phos had the gale to grin at him.

"Oh," Yagi let out. "How was it then?"

Phos' smile morphed into a childish scowl. "Disgusting."

At least, Shouta told himself as he sat back at his desk after properly scolding Phos like the child they weren't supposed to be, Hizashi hadn't been there. Shouta wasn't sure Phos would have survived both the flashbang and his friend's scream of surprise and horror.