Hello, sorry for bursting into this thread,
You should have been made aware from your employee (Nico) that the tiles that were already made, were not made for him or for free use
It was I that created them as a WIP some years ago, please send your employee my regards and suggestion to refrain from using my intellectual property, I told him many times to not do this, but since its not yet worth it to take legal actions, all I can do is telling him.
He should stop trying to spend as little resources as possible
You are definitely not at fault, but it would be delightful if his developer(s) created the tiles from scratch
He is free to contact me though to buy them

I like the various attempts of creating crubblestone tiles though, your recent edits look a little blurred though
You are free to imitate the style and colorvalues though, but not in that very composition
In interest of your progression of improvement, you can continue creating sprites that revolve around imitating the style, once the rest of the environment is made, you could create new cliffs and whichever that would then match your style more properly and transmit a better homogenic feeling (as stated above)