Why do some rocks glow under blacklight?

They are fluorescent, meaning that they absorb a wavelength of light and release it as a different wavelength. All objects reflect light, but fluorescent minerals reflect it in different wavelengths. UV (black) light isn't a wavelength visible to the human eye. That's why a room with UV light appears dark. The fluorescent minerals reflect it back as visible light. That's how blacklight posters work as well.

Minerals that they contain

methane