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How to clean quartz crystals
Use an old toothbrush.
If you have just a few quartz crystals or crystals that are only slightly tarnished by clay, you can use an old toothbrush to clean them off. Simply wet the toothbrush with water and scrub the quartz crystals in your hand.
You can even use scouring powder and a steel brush for tougher stains, as quartz is a very durable material. Dry the quartz crystals in the sunlight.