make a material with false depth

Hi, I’m trying to create something like this for an enemy in my game.

As you can see in the image, the object in this case is an ice-covered shield that reveals part of the shield that was originally.

My enemy is something similar, a combination of crystal and rock that merges in certain parts and the ideal would be to be able to see the rock under the crystal.

Do you have any suggestion or tutorial that could help me achieve something like this?

Thanks!