Hey,
I am currently looking for a way to see blocks through other blocks, because I want to make a mod which should allow the player to craft an item (e.g. with diamonds) to find blocks of the same kind (in this example diamonds ore).
The problem is that I don't know how to render the blocks at the moment, so far I'm so that crafting the items etc works, but I'm just hanging on rendering the blocks. I know that there is an event called RenderWorldLastEvent but I don't know how to find a block there (in this case a diamond ore) and make a box around it.

Bump 

The closest I've ever come to doing something like this, I used particle effects.

https://github.com/Draco18s/ReasonableRealism/blob/1.12.1/src/main/java/com/draco18s/ores/client/ProspectorParticle.java

Thanks, but I think your code is just to display the particles around the block. My main problem is to first find the corresponding blocks around the player and after this to display the particles / other effects around the block. Now that I know how the particles would work, I would need some way to find the block at all. 

There is no auto-magical way of getting a specific block in the world.
Use BlockPos.getAllInBox using two opposite corners to get an Iterable<BlockPos> containing all BlockPos within that area.

You then need to iterate through each element, and check if the block at that position is the block you want.
Do keep in mind to limit the area you want to check, as scanning in 3-dimensional space gets exponentially larger the further out you scan.