Thank you for your inquiry. Size is of importance for some crystals, but not for high vibration crystals like Phenacite. All pieces of Phenacite are connected and carry the energy of other Phenacite across the globe. I am still getting acquainted to my piece of Phenacite, but it seems similar to Moldavite in that it accelerates great change. I've had small pieces of Moldavite rock my world and sat with large chunks that didn't seem to alter my energy nearly as much.
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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 complement to other therapies and not as a replacement for regular medical care. The information offered here is based on a combination of my own personal experiences and that of others on using crystals for healing. My hope is that you will let your own intuition guide your path in working with crystals as well.