The sometimes unpredictable Rainbow Serpent (in contrast to the unyielding sun) replenishes the stores of water, forming gullies and deep channels as the Rainbow Serpent slithers across the landscape. In this belief system, without the Serpent, no rain would fall and the Earth would dry up. In other cultures, the Serpent is said to come to stop the rain. In addition to the identification with the rainbow, the Serpent is also identified with a prismatic halo around the moon that can be regarded as a sign of rain. The Rainbow Serpent is sometimes associated with human blood, especially circulation and the menstrual cycle, and considered a healer. Thunder and lightning are said to stem from when the Rainbow Serpent is angry, and the Serpent can even cause powerful rainstorms and cyclones. Quartz crystal and seashells are also associated with the Rainbow Serpent and are used in rituals to invoke it. The identification with quartz crystal presumably results from its prism-like appearance.