A fellow soartex user! HIGH FIVE!!!Hmm yeah slate is a bit tricky. I think it only spawns in river biomes. I am getting none of it in the mining age I posted earlier that includes ocean biomes. Slate, Hornfell and Onyx are the only ones it doesn't produce, but Onyx only spawn near lava afaik and there is on design no lava, so
But the Mariculture Automatic Dictionary Converter can change one type to another so it is not that huge of a problem. And I don't see that as that big of a cheat. Its just building materials.
But back to Slate, in Soartex slate has a rather distinct texture to it:
And if you look carefully it doesn't actually tile all that well. So dont think I will be using that. But yeah colour scheme is somewhat in the right direction.