Beryllium

Emerald ore is very unbalanced.  First, you must find an extreme hills biome.  Then, you must dig to the depths of the Overworld, in search of the rarest Overworld ore.  Finally you've found it, and you have one emerald in your possession.

...Or you could just trade with some villagers.

This imbalance needs to be fixed.  There is nearly no reason to mine for emerald ore.  So my idea is this:

If you mine emerald ore with Silk Touch, it'll drop its ore block.  This can be smelted in a furnace to obtain beryllium.  (In real life, beryllium is a rare, lightweight, toxic metal found in emeralds.)

A Smithing Table can be used to upgrade diamond armour with four beryllium, and tools with one.  Beryllium armour will not be stronger than Netherite, but will come with certain buffs that will make it useful.

Beryllium boots give you speed and Feather Falling, literally making you "light on your feet."  Leggings give you jump boost, and chestplates would have Thorns, as beryllium is toxic (I'm not sure about the helmet, though).  Weapons and tools would be lighter, and have a faster recharge.  Swords could have Sweeping Edge.  Pickaxes could have Silk Touch.  All of them would do more damage, and have a small chance of giving Poison.

To make up for these buffs, beryllium would be as protective as diamond, but not as durable.  Beryllium would add variety to the game, as players would have to choose certain weapons.  It would also give emerald ore a use.