I know it can be ignited by spark but he means an explosive ignited by only spark. Aka what c-4 is supposed to be but isn't.

just use rubidium and c4, the rubidium will carry spark to c4 and the c4 will explode the rubidium, vola.

EXAMPLE:
LoadSaveBlock(19254);
https://powdertoy.co.uk/View.html?ID=19254
hope it helps

Maybe he means an explosive that you can directly apply spark to (e.g. metal) and have it explode.

But it doesn't explode when you do.