Most pencil artists begin with using just graphite. But graphite, by it's very nature is incapable of producing extremely dark shading and when you try, it becomes
very reflective. Graphite pencil's limited value range, combined with this distracting reflectiveness can kill the realism you are trying to achieve. If you are attempting to put in extremely dark values, you are probably trying to represent an absence of light. The last thing you would want to do is have graphite shining in those areas that can look as bright as some of the highlights.
Tutorial by JD Hillberry