My idea is to use the Emerald to reinforce the armor and add a new effect to the player.
Through the anvil it would be possible to apply to the armor the emerald embedded in it.
For example, if we put an emerald on a helmet, the one just put on the head will produce a dynamic light effect;
Or if placed on a chestplate creates a protection effect and the addition of 3 hearts, etc.
I hope you enjoyed it and I apologize for any mistakes.
cordial greetings, GentylMy.