In HarvestLevelNamesOverride.cfg I used the following format:

HarvestLevelNames {
S:Level0=Stone
S:Level1=Copper
S:Level2=Bronze
S:Level3=Iron
S:Level4=Diamond
S:Level5=Obsidian
S:Level6=Ardite
S:Level7=Cobalt
S:Level8=Adamantine
S:Level9=Atlarus
}

All levels were changed to the corresponding text, with the exception of level 4, which remained as REDSTONE, which normally won't be an issue but in my modpack, Redstone is actually level 3 or IRON level.

Only tinker-materials are supported. The comment does not really explain this properly.
It'll be somewhat remedied by 9d8a102