Natural Rock Crystal, While various gems are used to enhance the benefic power of various planets, yantras are used to control the malefic effect of various planets. The work of Navgrali Yantra, like the work of Navrattan rings and pendants, is to remove the malefic effect of various planets and bring a harmony in their working. This is hung at the entrance of the house or in the place of worship. It brings the vibrations of all the nine planets in harmony.