Dear K, Thank you for your inquiry. A crystal turns cloudy or dark when it has absorbed too much energy. This crystal should not be used again for cleaning purposes, because it has "burnt" out. Although not all crystals do this, some can, because they can't handle the amount of energy they are absorbing. To help this crystal heal you can place it in dirt or even a bowl of water with salt (if it can be immersed in water) to be cleansed. If in dirt leave for two to 3 weeks. For the water, leave it for two to three days. Here is a link on how to clean your crystals. Then I would place it in moonlight and sunlight (if it doesn't fade in the sun) for a month. After that I would use it in grids or plants, but not for any cleansing work.
I hope that these recommendations can be of assistance and that crystals grace your journey with love and light.
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Disclaimer – Crystals are not a medical treatment and should not be used as a replacement for medical care. Please consult your healthcare specialist for your particular needs.