Smoky quartz is a grounding and protective stone that helps against anxiety, especially performance anxiety, nightmares and depression. It helps you to think clearly and realistically and increases concentration. It gives you the focus you need to turn your ideas into reality. It works against stress and helps you to relax, to let go and to accept.
Smoky quartz can discolor when left in bright sunlight, but it can be cleansed and charged in every other way. Here I explain more about cleansing and here about charging crystals. :)