I did mention this ages ago, in Farming testing in response to you asking “Anything else?” there too. 
Mosaic, marble, refined gravel and wicker?
I understand that not being able to spray them may have to do with making them keep their value for their tint, though it won’t be any different from spraying 45 refined rock instead of spraying 50 finished mosaic, except I’ll have to use 5 more spray cans if I want to spray the finished result. The end result is the same.