Clear quartz is good for amplifying energies but since the OP is specifically asking for something to aid with stress and protection from negativity I think they would benefit more from the ability of the black tourmaline to absorb and ground negative energies. You definitely don't need a diamond or a ruby 
Not to say clear quartz isn't useful in this case. As you said it can be used for many purposes, such as adding energy to a meditation with the intent to reduce stress.
Not to say clear quartz isn't useful in this case. As you said it can be used for many purposes, such as adding energy to a meditation with the intent to reduce stress.