This is a list of crystals and stones that are not suitable for placing in water, especially for crystal elixirs. From: Gem Water: How to Prepare and Use Over 130 Crystal Waters for Therapeutic Treatments by Joachim Goebel and Michael Gienger.
Alunite: Non toxic but soluble in water
Anglesite: Toxic and slightly soluble in water
Arsenopyrite: Potentially toxic
Bunsenite: Toxic; allergen; avoid skin contact
Calomel: Possibly hazardous to health
Chalcanthite: Harmful; easily soluble in water
Cinnabar: Very toxic!
Cinnabar-Opal: The cinnabar stored in the opal is toxic