I'm trying to integrate Atum into the overworld by very rarely generating an Oasis in deserts, which works for the most part.

However, instead of sand I'm getting stone blocks generated:

This happens with the others, like Sand Planes, too. I hope this can somehow be fixed, because I'm planning to spawn partial portal frame structures in the overworld to introduce players to Atum 🐢

Thanks for your report!
Could I get you to send your Climatic Biomes config?

Sure, but I've updated the values since opening the issue (and probably need to tweak more in the future):

biome(minecraft:desert)
biome(minecraft:desert)
biome(minecraft:desert)
biome(minecraft:desert)
biome(minecraft:desert)
biome(minecraft:desert)
seed(minecraft:desert, 250, atum:dead_oasis)
seed(minecraft:desert, 1000, atum:oasis)
biome(minecraft:desert_hills)
biome(minecraft:desert_hills)
biome(minecraft:desert_hills)
biome(minecraft:mutated_desert)
noise(minecraft:mesa, 5, minecraft:mesa_clear_rock)
noise(minecraft:mesa, 5, minecraft:mesa_clear_rock)
noise(minecraft:mesa, 5, minecraft:mesa_rock)
noise(minecraft:mesa, 5, minecraft:mesa_rock)
noise(minecraft:mesa, 5, minecraft:mutated_mesa_rock)
noise(minecraft:mutated_mesa_clear_rock, 5, minecraft:mutated_mesa)

(note that I merely added the atum related lines)

I didn't put this into the /custom/ folder, but the /Minecraft/Desert.cfg file directly for now (which shouldn't make any difference in terms of generation).

I think you can just set the seed value to something low to make it generate more, if you're going to test.

Cheers.
I hopefully have time to take a look at this later. Quite busy today.

Made a workaround for it. Since it's a very minor thing I'm not gonna release a new version for just that.
But you can go grab the fix from Jenkins here: http://girafi.dk/jenkins/job/Team%20Metallurgy%20Mods/job/Atum%202/job/Atum%202%20Jenkins/8/

Hey, awesome! Thanks for the quick fix. 💃