Some of you might have seen my last attempt at making a greenslate wall, if not check it out below:

Most of you agreed that it doesn't look like a slate wall because it lacks all those horizontal lines which are a characteristic for slates. So following your advice I've managed to achieve this:

You can see all the "layers" of the slate here, so I guess it looks better than the last bunch of stones. I will combine those two methods to create something similar to my reference picture you can check out here:
Reference picture
it has stones cut in various directions, which looks very nice and should look even better when recreated in 28mm scale. I will also add different hight of stones, to make the wall look even more realistic. It can be a bit tricky to add all those horizontal lines on 1mm thick pieces, but all the work should be worth it.
So what do you think, does this look like a slate, or not??
Thanks in advance for your comments and suggestion,
take care,
dsc.