Chalcanthite is a is a richly colored blue-green water-soluble copper sulfate 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. 

This specimen is from the Poland, and is most likely synthetically grown. Chalcanthite is highly toxic due to it's water soluble nature, sometimes referred to as "the blue death".