Do NOT put crystals anywhere unless you know there is good energy present, for crystals amplify all energies, good and bad. If you have a plant in a toxic energy field then the crystal will just magnify the toxic energies and make life that much more difficult for whatever or whoever is near the crystal.
Most people who collect crystals make the assumption that they are good for the environment and promote good energies, but this is not true. They magnify the good and the bad energies, so WHERE you place them is vital to their good workings. If you have a crystal garden (a group of many crystals together), then place this garden in the middle of a 16-emitter which might be made of crystals, stones, beads, wood, pine cones or whatever similar objects you decide upon. Below is a 16-emitter made of crystals with a plant atop it.

While crystals may be expensive when purschased individually, they are quite reasonably priced (approximately $0.13 each for a 1" crystal) when buying in bulk from such a place as Jim Coleman's Mine.
Egyptian Feng Shui - Body Psychic Readings
Egyptian Feng Shui - Circular 16-emitter
Egyptian Feng Shui - Pets Like Good Energies Too
Giving Your Plants Some Good Energies
Layering possibilities: General Layering Emitters, See Through Layering Emitters, and Large Corner Layering Emitters
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