Chalcanthite

Chalcanthite is a water soluble copper sulfate. As chalcanthite is a copper mineral, it can be used as an ore of copper. However, its ready solubility in water means that it tends to crystallize, dissolve, and recrystallize as crusts over any mine surface in more humid regions. Therefore, chalcanthite is only found in the most arid regions in sufficiently large quantities for use as an ore.

Secondarily, chalcanthite, due to its rich color and beautiful crystals, is a sought after collector's mineral. However, as with its viability as an ore, the solubility of the mineral causes significant problems. First, the mineral readily absorbs and releases its water content, which, over time, leads to a disintegration of the crystal structure, destroying even the finest specimens. It is critical to store specimens properly to limit exposure to humidity. Second, higher quality crystals can be easily grown synthetically, and, as such, there is a concern that disreputable mineral dealers would present a sample as natural when it is not.

Chalcanthite's blue color is one of its most notable features, but it is insufficient in identification. Other useful tests include associated minerals, crystal habit, solubility and subsequent coloring of the water blue, and taste. A special note on tasting chalcanthite, however: chalcanthite has a sweetly metallic taste, but taste testing should not be done haphazardly. The specimen should be never touched with the tongue, as chalcanthite is poisonous. The liquid from chalcanthite will also stain skin blue for several days.

Information above from http://en.wikipedia.org/wiki/Chalcanthite

Most of the Chalcanthite sold to collectors is laboratory grown and presents an excellent electric blue display specimen for your cabinet. Chalcanthite should always be kept inside a cabinet to prevent young children from reaching out and touching the "pretty blue rock"