Thank you for your inquiry!
To cleanse and purify your drinking water I would recommend using Tumbled Clear Quartz, Herkimer Diamonds (a variety of clear quartz), or Tumbled Smokey Quartz. Any and all of these crystals will hold a program well and can be charged to purify your water. Just be sure to cleanse them periodically and initially after purchase.
I would put them out in sunlight and moonlight for the cleansing and recharging. If this is not possible then I would recommend sage for cleansing, or place them around a salt lamp for a week. An alternate to sunlight and moonlight would be to put them on a copper plate or under a copper pyramid to charge.
Charging and Programming your Crystals:
To activate the crystal once it has been cleaned, you can hold it in your hands. While holding it imagine white light coming through your hands. You can then blow on the crystal, asking it to help purify and cleanse your water. Imagine your intent flowing from you and into the crystal. You could also write your desire for it to purify your water on a piece of paper and place the crystal on that piece of paper in the sunlight for a few hours.
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I must share the standard disclaimer that I am not a doctor and cannot give out medical advice. Crystals for healing should be used as a compliment to other therapies and not as a replacement for regular medical care.