Coffinite is a uranium bearing mineral that occurs as black incrustations, dark to pale-brown with a brittle to conchoidal fracture. The hardness of coffinite is between 5 and 6.  A large percentage of the Earth's uranium supply is contained in coffinite deposits. 

X-ray powder patterns from samples of coffinite allowed geologists to classify it as a new mineral in 1955.  Coffinite was first discovered in sedimentary uranium deposits in the Colorado Plateau region, but has also been discovered in sedimentary uranium deposits and hydrothermal veins in many other locations. Samples of coffinite from the Colorado Plateau were found with black fine-grained low-valence vanadium minerals, uraninite and finely dispersed black organic material.

This particular piece finds its way to me from Elias mine in Norway.
 
Using GMC600+ this sample emits about 40 micro sieverts per hour, but only about 1.8 from behind it's plastic container so the gamma emissions of this sample are quite low.