I am building a world with a small island nation trying to defend themselves against a large continent with medieval (1200-1300) level technology. They lack metal and use obsidian as weapons (like the aztec). How could they feasibly synthesise an explosive and how would they go about doing so?

I am building a world with a small island nation trying to defend themselves against a large continent with medieval (1200-1300) level technology. They lack metal and use obsidian and stone for weapons and tools, similar in technology level to the Aztec around 1500. How could they feasibly synthesise an explosive and how would they go about doing so?