Natural (not heated) smoky quartz is a great earthing stone with high vibration, works well to relieve all discomfort in lower abdomen, opens and balances root and sacral chakras. Decreases insomnia and hyperactivity, also sorrow, hopelessness and depression.
Flat stone suits the best for healingwork, because it covers the chakra and the contact surface are is the largest. Also it will not fall off the body and is large enough to draw forth energetic and physical changes. Flat stones are also the best to carry in your pocket.