Torbernite, also known as chalcolite, is a relatively common mineral.  It is a hydrated green copper uranyl phosphate containing uranium oxide, which is where it gains its radioactivity. 

This particular piece finds its way to me from the Congo, it came out of the Musonoi mine.
 
It is very similar to Autunite which is a Hydrated Calcium Uranyl Phosphate, also containing Uranium Oxide.  A "hydrated" crystal means part of the structure is made up of water, which can dehydrate changing the chemical structure over time. 

Torbernite gets its green color from copper, and does not fluoresce under UV unlike Autunite, which does fluoresce under UV. 

The chemical formula of Torbernite is similar to that of Autunite in which a Cu2+ cation replaces a Ca2+ cation - Torbernite contains copper, Autunite contains Calcite. 
 
Torbernite tends to dehydrate to metatorbernite, which can cause the crystal to break down and become powdery.  Inhaling the dust is dangerous due to the radioactive nature of the mineral, but don't worry this specimen is in a case. 

Using GMC600+ this sample emits 150 micro sieverts per hour of radiation, or roughly the equivalent of a dental x-ray every 4 minutes.  Don't let this alarm you, the plastic case you see blocks all of the Alpha and Beta radiation and those readings are only on contact with the mineral, observing with some air between you and the specimen brings the levels down to quite a safe range, you are fine.