Java Edition:Item Transmutation

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Item Transmutation refers to a set of methods usable to change the data or damage values of items to a desired value, analogously to Block Transmutation.

Currently known item transmutation methods, unlike block transmutation methods, are extremely limited, preventing access to a huge number of items. For example, spawn eggs for several exotic mobs, non-mob entities and mobs added in 1.2 and beyond are not known to be obtainable, but item transmutation would allow them to be obtained among other things.

This only refers to methods of changing numerical metadata - other item modifications such as NBT changing are not included unless they allow for unintentional metadata changes as well.

Brewing Stand Transmutation

Brewing Stand Item Transmutation is a method of transmutation that can set metadata values of items to 0. It cannot set them to any other values.

Via item combination

Many items which cannot be obtained with invalid data values normally can allow for conversion of existing data values into the desired state provided that at least one of that invalid data value is obtained beforehand. Examples include invalid data value seeds. This method can be both a blessing and a trap, as it can revert invalid states to valid, resulting in the effective destruction of the discontinued data value.