It is possible to dehydrate CuSO4 5H20 by heating it. These bright blue crystals transform to a pale green or gray-white powder as they dehydrate. However, the reaction is reversible by dissolving the CuSO4 in water and allowing crystals to grow as the water of solution evaporates. The anhydrous form is highly hygroscopic and also reverts to CuSO4 5H20 on standing.
The most common occurrence of CuSO4 in nature is as the pentahydrate, as a mineral scientists call chalcanthite; however, it exists in three other rare forms. Chalcocyanite is the anhydrous form, bonattite is the trihydrate and boothite is the heptahydrate. The water solubility of these minerals makes them unsuitable for use as gemstones.
Copper sulfate is an effective agricultural fungicide. It is a common swimming pool additive due to its properties as an algaecide. It preserves wood in pressure treated lumber. About.com recommends purchasing the blue crystal form, which is available in home improvement stores as a septic tank additive for preventing root growth in septic lines, to experiment with crystal growing at home.
Although manufacturers often include copper sulfate in children's chemistry kits, it is toxic and not appropriate for small children. While the salt induces vomiting, doctors consider it too toxic for use as an emetic.