It's funny - you pick up one of these 'rocks' only to find it's like solidified mud and is kinda 'squidgy'. Or then again, it might break apart and reveal an amazing find.
That said, all ones I pick up are mud...
Colour certainly made geological field photography easier (geology was my first work field before I transitioned to IT), but judicious use of filters could do a lot with contrast. Yellow/orange/red and green help with most rock types for illustrative purposes.