If you are looking for 1.19.3, ores should be done completely in jsons. A helpful tool to know is: [[https://misode.github.io/worldgen/feature/|Configured Features]] and [[https://misode.github.io/worldgen/placed-feature/|Placed Features]]

====== Generating Custom Ores [1.19.3+] ======
A lot of mods add their own ores, and you'll need a way to place them in existing biomes for players to find. In this tutorial, we'll look at adding ores to existing biomes. There are 2 steps that are required to add ores to biomes. 
  * Make worldgen Features in JSON that define how and where the ore block is spawned.
  * Use [[https://github.com/FabricMC/fabric/pull/1097|Biome Modification API in Fabric API]] to add your feature to biomes.

To simplify, we use vanilla end rod as ore, because it can be easily found and seen underground in spectator mode.

==== Adding to the overworld biomes ====
In this section, our goal will be spawning the ore in the overworld.

First, create two new JSON files in your mod's data directory:

<code JavaScript src\main\resources\data\tutorial\worldgen\configured_feature\ore_custom.json>
{
  "type": "minecraft:ore",
  "config": {
    "discard_chance_on_air_exposure": 0.0,
    "size": 12,
    "targets": [
      {
        "state": {
          "Name": "minecraft:end_rod"
        },
        "target": {
          "predicate_type": "minecraft:tag_match",
          "tag": "minecraft:stone_ore_replaceables"
        }
      },
      {
        "state": {
          "Name": "minecraft:end_rod"
        },
        "target": {
          "predicate_type": "minecraft:tag_match",
          "tag": "minecraft:deepslate_ore_replaceables"
        }
      }
    ]
  }
}
</code>

This Configured Feature tells the game the size of the ore veins, what fraction of the ore blocks should be removed due to air exposure, and, importantly, which blocks the ore block should be allowed to replace. Notice how there are two ''target'' objects in the ''targets'' array: one for ore in stone, and another is the ore in deepslate. To simplify it, both we use end rod in vanilla games.

<code JavaScript src\main\resources\data\tutorial\worldgen\placed_feature\ore_custom.json>
{
  "feature": "tutorial:ore_custom",
  "placement": [
    {
      "type": "minecraft:count",
      "count": 20
    },
    {
      "type": "minecraft:in_square"
    },
    {
      "type": "minecraft:height_range",
      "height": {
        "type": "minecraft:trapezoid",
        "max_inclusive": {
          "absolute": 70
        },
        "min_inclusive": {
          "absolute": -24
        }
      }
    },
    {
      "type": "minecraft:biome"
    }
  ]
}
</code>

This Placed Feature tells the game how many ore veins to place in a chunk, what shape the ore veins are placed in, and at what y-levels the ore veins are placed in. Note that the first line of the Placed Feature references the Configured Feature that was created a moment ago. 
 
Writing these JSON files by hand is strenuous and inefficient. Note that the bulk of the work can be skipped by using tools like this [[https://misode.github.io/worldgen/feature/|Configured Features generator]] and this [[https://misode.github.io/worldgen/placed-feature/|Placed Features generator]]. Alternatively, use compressed file tools to open your vanilla Minecraft ''.jar'' and use the vanilla files as a reference. 

Now that our data is created, it's time for code! Thanks to the changes in Minecraft 1.19.3, adding ore to world generation requires much less Java code. All we need to do is register the feature, then use the Fabric Biome Modification API to tell Minecraft which dimension to generate the ore in (and during which stage of world generation to generate the ore).

First, at the class level, let's create a new ''RegistryKey'' at the class-level to store our ore. 

<code Java src/main/java/net/fabricmc/example/ExampleMod.java>
public class ExampleMod implements ModInitializer {
	
	public static final RegistryKey<PlacedFeature> CUSTOM_ORE_PLACED_KEY = RegistryKey.of(RegistryKeys.PLACED_FEATURE, Identifier.of("tutorial","ore_custom"));

	@Override
	public void onInitialize() {

		//Your other code here...

	}
}
</code>

Notice 2 things:
  * ''RegistryKey'' is a generic. We give it the type of PlacedFeature.
  * our identifier uses the name "ore_custom" which is the filename of our Placed Feature JSON file. We don't need to give it the Configured Feature, because the Placed Feature stores the name of its corresponding Configured Feature.

Now to add the Feature to a biome:

<code Java src/main/java/net/fabricmc/example/ExampleMod.java>

@Override
	public void onInitialize() {
	
		//Your other code here...
		BiomeModifications.addFeature(BiomeSelectors.foundInOverworld(), GenerationStep.Feature.UNDERGROUND_ORES, CUSTOM_ORE_PLACED_KEY);
	}

</code>

==== Testing ====

To test your new ore, generate a new world. You can also open existing world but have to go to new chunks. In this example of end rod, the ores can be directly seen underground in spectator mode.

==== Adding to the Nether or End ====

Adding your ore to the other dimensions is very similar to adding ore to the Overworld. This section assumes you have already created an ore block for the Nether and End.

As with the Overworld, we begin with the JSON files. Using vanilla's nether gold ore as an example, our Placed Feature will look something like this:

<code JavaScript src/main/resources/data/tutorial/worldgen/placed_feature/ore_custom_nether.json>
{
  "feature": "tutorial:ore_nether_custom",
  "placement": [
    {
      "type": "minecraft:count",
      "count": 20
    },
    {
      "type": "minecraft:in_square"
    },
    {
      "type": "minecraft:height_range",
      "height": {
        "type": "minecraft:uniform",
        "max_inclusive": {
          "below_top": 10
        },
        "min_inclusive": {
          "above_bottom": 10
        }
      }
    },
    {
      "type": "minecraft:biome"
    }
  ]
}
</code>

Notice how the only real difference is that the height ranges are described differently, and the height type is ''uniform'' instead of ''trapezoid''. Trapezoid shapes and absolute height ranges are still valid in the Nether, we are simply following vanilla Minecraft's game design here. Your mod may use whichever you prefer.

As before, we add a Configured Feature as well.

<code JavaScript src/main/resources/data/tutorial/worldgen/configured_feature/ore_nether_custom.json>
{
  "type": "minecraft:ore",
  "config": {
    "discard_chance_on_air_exposure": 0.0,
    "size": 20,
    "targets": [
      {
        "state": {
          "Name": "minecraft:end_rod"
        },
        "target": {
          "block": "minecraft:netherrack",
          "predicate_type": "minecraft:block_match"
        }
      }
    ]
  }
}
</code>

In the Configured Feature, remember that the ''Name'' key is the name of your ore block, not the name of your Placed Feature.

Finally, back in our Java code, right after our other BiomeModification line,

<code Java src/main/java/net/fabricmc/example/ExampleMod.java>
		BiomeModifications.addFeature(BiomeSelectors.foundInNether(), GenerationStep.Feature.UNDERGROUND_ORES, CUSTOM_ORE_PLACED_KEY);
</code>

Extrapolating this process to add ore to the End is left as an exercise for the reader.

====== Generating Custom Ores [1.18.2 / 1.19.2] ======
A lot of mods add their own ores, and you'll need a way to place them in existing biomes for players to find. In this tutorial, we'll look at adding ores to existing biomes. There are 2 steps that are required to add ores to biomes.
  * Make a ConfiguredFeature. This defines how your ore block is spawned.
  * Use [[https://github.com/FabricMC/fabric/pull/1097|Biome Modification API in Fabric API]] to add your feature to biomes.

Note that the Biome Modification API is marked as experimental.
If the API doesn't work, consider using [[?rev=1599388827|the mixin version]].

We'll assume you've already created your own ore block at this point. Wool block will serve as our replacement throughout this tutorial. Replace references to wool with your ore when appropriate.

==== Adding to the overworld biomes ====
In this section, our goal will be spawning the ore in the overworld.

We need to create a ConfiguredFeature.
Make sure to register your ConfiguredFeature at ''onInitialize''.
Feel free to change the values to suit your mod.

<code java>
public class ExampleMod implements ModInitializer {
  private static ConfiguredFeature<?, ?> OVERWORLD_WOOL_ORE_CONFIGURED_FEATURE = new ConfiguredFeature
      (Feature.ORE, new OreFeatureConfig(
          OreConfiguredFeatures.STONE_ORE_REPLACEABLES,
          Blocks.WHITE_WOOL.getDefaultState(),
          9)); // vein size

  public static PlacedFeature OVERWORLD_WOOL_ORE_PLACED_FEATURE = new PlacedFeature(
      RegistryEntry.of(OVERWORLD_WOOL_ORE_CONFIGURED_FEATURE),
      Arrays.asList(
          CountPlacementModifier.of(20), // number of veins per chunk
          SquarePlacementModifier.of(), // spreading horizontally
          HeightRangePlacementModifier.uniform(YOffset.getBottom(), YOffset.fixed(64))
      )); // height

  @Override
  public void onInitialize() {
    Registry.register(BuiltinRegistries.CONFIGURED_FEATURE,
        new Identifier("tutorial", "overworld_wool_ore"), OVERWORLD_WOOL_ORE_CONFIGURED_FEATURE);
    Registry.register(BuiltinRegistries.PLACED_FEATURE, new Identifier("tutorial", "overworld_wool_ore"),
        OVERWORLD_WOOL_ORE_PLACED_FEATURE);
    BiomeModifications.addFeature(BiomeSelectors.foundInOverworld(), GenerationStep.Feature.UNDERGROUND_ORES,
        RegistryKey.of(Registry.PLACED_FEATURE_KEY,
            new Identifier("tutorial", "overworld_wool_ore")));
  }
}
</code>

=== Result ===
Always create a new world when you check the world generation result. You should see wool spawning in the overworld. You can use the command below to remove stone blocks surrounding you.
<code>
/fill ~-8 0 ~-8 ~8 ~ ~8 minecraft:air replace minecraft:stone
</code>

{{tutorial:ores.png?800}}

==== Adding to the nether biomes ====
In this section, based on the code in the previous section, we will add the ore to the nether biomes.

We need to replace ''OreConfiguredFeatures.STONE_ORE_REPLACEABLES'' with ''OreConfiguredFeatures.NETHERRACK''
because the ore has to replace different blocks in the nether than in the overworld.

<code java>
public class ExampleMod implements ModInitializer {
  private static ConfiguredFeature<?, ?> NETHER_WOOL_ORE_CONFIGURED_FEATURE = new ConfiguredFeature
      (Feature.ORE, new OreFeatureConfig(
          OreConfiguredFeatures.NETHERRACK, // we use OreConfiguredFeatures.NETHERRACK here
          Blocks.WHITE_WOOL.getDefaultState(),
          9));

  public static PlacedFeature NETHER_WOOL_ORE_PLACED_FEATURE = new PlacedFeature(
      RegistryEntry.of(NETHER_WOOL_ORE_CONFIGURED_FEATURE),
      Arrays.asList(
          CountPlacementModifier.of(20),
          SquarePlacementModifier.of(),
          HeightRangePlacementModifier.uniform(YOffset.getBottom(), YOffset.fixed(64))));

  @Override
  public void onInitialize() {
    Registry.register(BuiltinRegistries.CONFIGURED_FEATURE,
        new Identifier("tutorial", "nether_wool_ore"), NETHER_WOOL_ORE_CONFIGURED_FEATURE);
    Registry.register(BuiltinRegistries.PLACED_FEATURE, new Identifier("tutorial", "nether_wool_ore"),
        NETHER_WOOL_ORE_PLACED_FEATURE);
    BiomeModifications.addFeature(BiomeSelectors.foundInTheNether(), GenerationStep.Feature.UNDERGROUND_ORES,
        RegistryKey.of(Registry.PLACED_FEATURE_KEY,
            new Identifier("tutorial", "nether_wool_ore")));
  }
}
</code>

==== Adding to the end biomes ====
In this section, based on the code in the overworld section, we will add the ore to the end biomes.

We replace ''OreConfiguredFeatures.STONE_ORE_REPLACEABLES'' with ''new BlockMatchRuleTest(Blocks.END_STONE)''.

<code java>
public class ExampleMod implements ModInitializer {
  private static ConfiguredFeature<?, ?> END_WOOL_ORE_CONFIGURED_FEATURE = new ConfiguredFeature
      (Feature.ORE, new OreFeatureConfig(
          new BlockMatchRuleTest(Blocks.END_STONE), // we use new BlockMatchRuleTest(Blocks.END_STONE) here
          Blocks.WHITE_WOOL.getDefaultState(),
          9));

  public static PlacedFeature END_WOOL_ORE_PLACED_FEATURE = new PlacedFeature(
      RegistryEntry.of(END_WOOL_ORE_CONFIGURED_FEATURE),
      Arrays.asList(
          CountPlacementModifier.of(20),
          SquarePlacementModifier.of(),
          HeightRangePlacementModifier.uniform(YOffset.getBottom(), YOffset.fixed(64))));

  @Override
  public void onInitialize() {
    Registry.register(BuiltinRegistries.CONFIGURED_FEATURE,
        new Identifier("tutorial", "end_wool_ore"), END_WOOL_ORE_CONFIGURED_FEATURE);
    Registry.register(BuiltinRegistries.PLACED_FEATURE, new Identifier("tutorial", "end_wool_ore"),
        END_WOOL_ORE_PLACED_FEATURE);
    BiomeModifications.addFeature(BiomeSelectors.foundInTheEnd(), GenerationStep.Feature.UNDERGROUND_ORES,
        RegistryKey.of(Registry.PLACED_FEATURE_KEY,
            new Identifier("tutorial", "end_wool_ore")));
  }
}
</code>