This guide will go over the optional Wish Volatile Fintbeetles in Hollow Knight: Silksong, including where to find all three enemies.
How to Find Flintbeetles
Volatile Flintbeetles is an optional Wish that appears on the Wish Board in Bone Bottom as you progress through the game. After you accept the Wish, three Flintbeetles are added to The Marrows, with glowing rocks lying around that can be used to track them.
While the intention is to interact with the glowing rocks and follow them to your targets, you can instead just go to their locations assuming you know them ahead of time. Below is a map of where to find all three enemies, without the need to manually track them.
Note that after you find the Flintbeetles, you'll have to actually fight them too. They'll jump around leaving explosions in their wake, or fling out explosive rocks. Naturally, they also explode when dying. After you kill them, make sure to pick up the Flintgems that they drop.
Flintbeetle Locations

Wish Reward
Heading back to the Wish Board in Bone Bottom will let you trade the 3 Flintgems for a Memory Locket. Memory Lockets are used in the Crest menu to add more tool slots to your Crests. Each Crest must be expanded individually, so you'll want to gather lots of Memory Lockets.
That's all for our guide on completing Volatile Fintbeetles! Make sure to check out more of our Silksong guides below.