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

Azurite: Harmful

Azurite-Malachite: Harmful

Azurite-Pseudomalachite: Harmful

Bunsenite: Toxic; allergen; avoid skin contact

Calomel: Possibly hazardous to health

Cerussite: Toxic

Chalcanthite: Harmful; easily soluble in water

Cinnabar: Very toxic!

Cinnabar-Opal: The cinnabar stored in the opal is toxic

Crocoite: Toxic 

Cuprite: Harmful

Durangite: Potentially toxic

Eclipse Stone: Limestone with orpiment, toxic

Eilat Stone: Harmful

Erythrite: Potentially toxic

Fiedlerite: Toxic

Fluorite, Antozonite variety: Potentially harmful

Galenite: Toxic and slightly soluble in water

Gaspeite: Harmful; allergen; avoid skin contact

Greenockite: Toxic

Halite: Not toxic in small quantities, but dissolves easily in water

Iron-nickel Meteorite: Harmful; allergen; avoid skin contact

Jamesonite: Potentially toxic

Lemon Chrysoprase: Potentially harmful; allergen; avoid skin contact

Lopezite: Very toxic! Hazardous even through skin contact

Malachite: Harmful

Millerite: Toxic; allergen; avoid skin contact

Minium: Toxic

Nickeline: Toxic; allergen; avoid skin contact

Olivenite: Potentially toxic

Orpiment: Toxic

Proustite: Potentially toxic

Psilomelane and Pyrolusite: Harmful

Pyromorphite: Potentially toxic

Rauenthalite: Toxic

Realgar: Toxic. Store in dark, securely locked place.

Scorodite: Potentially toxic

Sphaerocobaltite: Harmful; allergen; avoid skin contact

Stibnite: Harmful

Tetrahedrite: Potentially harmful

Ulexite: Non toxic but slightly soluble in warm water

Valentinite and Senarmontite: Harmful

Vanadinite: Toxic

Wulfentie: Potentially toxic