Mahagon obsidian has gentler energy than black obsidian. It brings forward the truth and helps to accept it. Mahagon obsidian brings out suppressed feelings from our unconciousness so we could acknowledge them and let go of them. Only use obsidians when you are sure that you are ready to look into your true self.
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.