Learn to code Minecraft mods

// ModItems.java (ours: Fabric docs pattern, MC 26.2)
public class ModItems {
    public static Item register(
            ResourceKey<Item> key,
            Function<Item.Properties, Item> factory,
            Item.Properties settings) {
        Item item = factory.apply(settings.setId(key));
        Registry.register(BuiltInRegistries.ITEM, key, item);
        return item;
    }

    public static final ResourceKey<Item> RUBY_KEY =
        ResourceKey.create(Registries.ITEM,
            ExampleMod.id("ruby_shard"));

    // stacks of 16 instead of the usual 64
    public static final Item RUBY_SHARD = register(
        RUBY_KEY, Item::new,
        new Item.Properties().stacksTo(16));

    public static void initialize() {
        CreativeModeTabEvents
            .modifyOutputEvent(CreativeModeTabs.INGREDIENTS)
            .register(tab -> tab.accept(RUBY_SHARD));
    }
}

Written with the classes and methods Fabric’s 26.2 documentation uses: ResourceKey, Item.Properties with setId, Registry.register and CreativeModeTabEvents. Imports are left out to keep it short; your editor offers to add them. Fabric notes that a damageable item always stacks to 1, so stacksTo only works on items like this one.

// ExampleMod.java: the main entrypoint (ours)
public class ExampleMod implements ModInitializer {
    public static final String MOD_ID = "example-mod";
    public static final Logger LOGGER =
        LoggerFactory.getLogger(MOD_ID);

    @Override
    public void onInitialize() {
        ModItems.initialize(); // loads the class
        LOGGER.info("Ruby shard registered");
    }

    public static Identifier id(String path) {
        return Identifier.fromNamespaceAndPath(MOD_ID, path);
    }
}

Fabric explains why initialize() can be empty and still matter: calling any method on a class makes Java load it, which runs its static fields, and that is where the item is registered.

// assets/example-mod/lang/en_us.json
{ "item.example-mod.ruby_shard": "Ruby Shard" }

// assets/example-mod/models/item/ruby_shard.json
{
  "parent": "minecraft:item/generated",
  "textures": {
    "layer0": "example-mod:item/ruby_shard"
  }
}

// assets/example-mod/items/ruby_shard.json
{
  "model": {
    "type": "minecraft:model",
    "model": "example-mod:item/ruby_shard"
  }
}

Three small JSON files, each in its own folder under src/main/resources. The texture is a 16x16 PNG named ruby_shard.png in assets/example-mod/textures/item. Replace example-mod with your own mod id everywhere.