ah. It's not pixelart, but this could be a good reference...
https://www.reddit.com/r/whatsthisrock/comments/1dilhxe/black_rock_found_in_a_river_deposition/
surprisingly, the colors captured are almost perfect shades of grey.
let's try something ... Agreed, there's an horrible realism clash here but see how the real object has no sort of "highlight zone on the top side in real world ... (okay, object was lit from the side .. maybe not the best example)
but its shape means that if there were a highlight area, it would be very thin because we go from vertical to horizontal very fast along that edge.
possibly #owlboy could come to the rescue ? (I had it in my printed barely readable #pixelstudy folder, but it comes from https://nitter.net/snakepixel/status/758402588329009153)
Makes me think that the big rock doesn't directly need to be side-view.