The Gem Knife uses emeralds to harvest ores from their ore block counter part. The Gem Knife uses the charge mechanic where the charge menu can be opened by using the left mouse button, and up to nine stacks or 576 emeralds may be inserted into it. Each Emerald inserted into the Gem Knife counts as two charges. The Gem Knife may be used on these ores and their deepslate/netherrack variants by using the right mouse button.
The Gem Knife belongs to the Watson collection and does have a prerequisite to unlock it's recipe. This prerequisite demands the player mine 24000 sand blocks to unlock the recipe. The Gem Knife has a cheap cost to craft requiring:
The Gem Knife doesn't require a full charge to activate. It can be used as long as there's a minimum of one emerald in the charge menu.
Non-emerald items placed into the charge menu will instead drop on the ground.
Emerald Blocks do not give it extra charges and will also drop on the ground.
The Gem Knife cannot move in the charge menu interface making it impossible to use itself as a charge
After a certain amount of emeralds have been added into the charge menu, using the Gem Knife will use 64 charges instead of just one.
This reverts back to one charge after even emeralds have been used as well.
The Gem Knife does not break the ore block but instead just spawns ore from it.
When on an odd numbered charge, upon opening and closing of the Gem Knife, it deletes a charge. This is due to one out of the two charges consuming the emerald. Upon opening the charge menu the Gem Knife sees that the emerald is not there so it updates it to fix the charge counter to the current amount of emeralds in the charge menu.
A player that wants to use the Gem Knife should try and find a method of producing Emeralds very cheaply due to the cost of Gem Knife.
The Gem Knife when used on iron and gold ore will not yield raw iron/gold but instead their nugget variants. This makes it not really worth the charge usage even if it doesn't break the ore block thanks to other methods of obtaining nuggets.
The Gem Knife is arguably best used on lapis lazuli and quartz ore due to cost efficacy compared to trading for them.
The Quartz Blossom is better than the Gem Knife if a player is trying to get a large amount of quartz blocks.