First Resonance Gem
The first resonance gem can be bought from Revus Sarvani for around 1000 gold.
Second Resonance Gem
The second resonance gem can be found at the end of Fahlbtharz.
In Fahlbthars, you can also find the Visage of Mzund. A unique helmet with the power to blow steam like a centurion.
Third Resonance Gem
The third resonance gem can be found during the main questline in Nchardak.
Once you reach this puzzle, go across the first two bridges and stop once you hit this wall.
Climb this wall and the gem will be at the top.
Fourth and Fifth Resonance Gem
The last two resonance gems can be found in the ruins of Kagrumez.
They are on this reaver.
P.S. Thats all of them!