I'd like to be able to force multiple ore veins to spawn at the same location. This has a ton of potential for creating interesting world generation. For example:

• A geode made by create two spheres of different materials

• A sphere of basalt with smaller veins of basalt around it, making it look like they broke off from the larger piece

• A large ore cloud of copper, with a smaller denser vein of copper blocks in the center.

Could this feature possibly be backported to 1.12.2 as well?

Alright, so I think the best way to approach this from a configuration point of view would be the following structure:

{
    "type": "multiple",
    "children": [
        {
            "type" "sphere"
            ...
        },
        ...
    ]
}

The root vein would have it's own spawning conditions (rarity, count, biomes, etc.). It would not specify any ore or stone requirements. Each sub vein would then sequentially be applied - including rarity, count (although these could be just set to always appear if the vein in question should always use all sub veins) - so that the ore veins are constructed predictably. I think this would satisfy the ability to create all the examples that you listed, in combination with other existing configuration options.

What minecraft version were you looking for?

I'm using 1.15 and would find this feature very useful

Added in faa22c4