There's a kind of hard to place rock that thinks it is special and needs special recognition. It's a rock that often 'sticks out' from the wall, or appears to be set too far back into the wall. Usually this is because its difficult to know where the 'plane' of its 'special' face actually is, and so setting it becomes a problem. While it may seem that this type of rock would best be put in a prominent place in your wall due to its special shape or interesting colour, it is usually not a good idea. If a rock is chosen and placed merely for its looks, disregarding its structural value, then it will be a rock that is basically in a 'race to the bottom' While others in the wall cooperate and become one tidy entity, these 'posers' may end up bringing their part of the wall down, visually, if not literally.
These high-maintenance rocks, the kind you'd actually like to cement in the wall, can come in other shapes too. There are the ones that are not very natural at all. They are more than likely guillotined, or specially quarried/chosen 'veneer' stones. A wall that incorporates a lot of these so-called 'tracers', the kind that are way too shallow or with their bed face showing ( no matter how special or beautiful they are ) will start to slump and fall apart before too long.