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World gen templates are used to make your custom blocks spawn in a newly generated world. Custom blocks spawn in veins of a random amount of blocks. They may only spawn in a set list of biomes of worlds. Gen templates do also have more specific options that this wiki page will be overviewing.

Basic gen template example

basic-template:
    replace: [STONE]
    chunk-chance: 0.7
    depth: 10=24
    vein-size: 5
    vein-count: 2

When a world is being generated, every chunk has a set chance to be selected in order to spawn a block (chunk-chance). You may also configure the depth at which your block will be placed. The replace option defines what vanilla blocks your custom block will replace. That means custom blocks cannot spawn midair so that custom ores can spawn inside stone walls just like vanilla ores.

vein-count is the amount of veins that will be generated inside a selected chunk, vein-size is the amount of custom blocks that will be generated in every block vein.

Specific options

Use the slime-chunk option so that your block may only spawn in slime chunks.

template-id:
    slime-chunk: true

Biome/World black/whitelist

By adding an entry to the worlds list, you can restrict any custom block to a specific set of worlds. If you use ! before the world name, the world whitelist will turn into a blacklist, and the custom block will spawn anywhere but in the specified worlds.

template-id:
    worlds:
    - world_nether # Just like nether quartz

The format is the same for biomes, just use biomes instead of worldsspigot docs

template-id:
    biomes:
    - moutains # Just like emerald