I play this mod and I know for certain what I said is true. I don't mean the giant boulders that are 2-4 blocks high and 2-4 blocks wide. I mean the small ones that have 3 stages that use to give iron. They are in fact in the burnt biome, as I've mined my coal and quite a bit of oil shale in the burnt biome. There are some nitrate in the forest biome, but it's more abundant in the snow biome. I should also note that, there's not always veins underneath the boulders.Yes, without a mod it is!And then all the boulders are made of stone!
Others did not come across!
In a burned biome there are only bonfires!
It's pretty simple. You beat on a boulder to see what it is. If it's what you want, dig down to the stone. If you find gravel, great, you have veins. If you don't find gravel, move on to the next boulder. This is what I do and I've had mines so plentiful I've filled up a chest with iron and one with lead. I had well over 20k gun powder made up from the coal and nitrate mines I had found as well.
Have a link to a pic of the type of boulder I'm talking about: https://images.app.goo.gl/EmmNWQNs9poxg7RdA
Apparently it's a pic from a mod that respawns boulders. So, if said author is here, sorry I "stole" your pic.