I am a bit confused with this mod and i am unable to find proper documentation.

I created a script as described in the wiki and copy-pasted it and replaced the variable name.
But when i start up the game, no custom materials can be found.

Do you need ContentTweaker for implementing custom materials?

Using:
Minecraft Version: 1.12.2
Forge: forge1.12.2-14.23.0.2486
BASE: 3.3.0

Also Using:
Binnie-Mods
Calculator
CommonCapabilities
Crafttweaker
Cucumber
CyclopsCore
Forestry
Forgelin
HardcoreDarkness
Industrialforegoing
IntegratedDynamics & Tunnels
JEI
mcjtylib
mysticalagriculture
QuantumStorage
RebornCore
refinedstorage
rftools
sonarcore
Techreborn
tesla-core-lib

Thank you in advance
Chenjox

As of 1.12, yes you need ContentTweaker for implementing Custom Materials.

The new script that can be used as an example for this can be found in ContentTweaker's WIki.

As a suggestion: I would notice my audience about the now necessary ContentTweaker on the libaries site to counteract confusion.

Thank you for your help

Correct. This is a really new change so we're still in process of making sure the documentation gets updated!