ExampleWorldGen.java

package com.example.examplemod;

import net.fabricmc.fabric.api.biome.v1.BiomeModifications;
import net.fabricmc.fabric.api.biome.v1.BiomeSelectors;
import net.minecraft.registry.RegistryKeys;
import net.minecraft.registry.tag.BiomeTags;
import net.minecraft.world.gen.GenerationStep;

/**
 * Adds a custom ore feature to all "is_overworld" biomes using Fabric's
 * BiomeModifications API. The underlying ConfiguredFeature/PlacedFeature
 * are defined as JSON datapack files (see comment at bottom);
 * BiomeModifications API unchanged across 1.21.7 - 1.21.11.
 */
public final class ExampleWorldGen {

    private ExampleWorldGen() {}

    public static void register() {
        BiomeModifications.addFeature(
                BiomeSelectors.tag(BiomeTags.IS_OVERWORLD),
                GenerationStep.Feature.UNDERGROUND_ORES,
                net.minecraft.registry.RegistryKey.of(
                        RegistryKeys.PLACED_FEATURE,
                        net.minecraft.util.Identifier.of(ExampleMod.MOD_ID, "ore_example_gem_ore")
                )
        );
    }
}

/*
 * data/examplemod/worldgen/configured_feature/ore_example_gem_ore.json
 * {
 *   "type": "minecraft:ore",
 *   "config": {
 *     "size": 9,
 *     "discard_chance_on_air_exposure": 0.0,
 *     "targets": [
 *       {
 *         "target": { "predicate_type": "minecraft:block_match", "block": "minecraft:stone" },
 *         "state": { "Name": "examplemod:example_block" }
 *       }
 *     ]
 *   }
 * }
 *
 * data/examplemod/worldgen/placed_feature/ore_example_gem_ore.json
 * {
 *   "feature": "examplemod:ore_example_gem_ore",
 *   "placement": [
 *     { "type": "minecraft:count", "count": 4 },
 *     { "type": "minecraft:in_square" },
 *     { "type": "minecraft:height_range",
 *       "height": { "type": "minecraft:trapezoid", "min_inclusive": { "absolute": -32 }, "max_inclusive": { "absolute": 32 } } },
 *     { "type": "minecraft:biome" }
 *   ]
 * }
 */