This strontianite is from the Cave-In-Rock Sub District, Hardin County, Illinois, USA. Dozens of different mines in the area have produced almost three dozen different minerals species including strontianite. It is a carbonate that contains the elements Sr, C and O. It commonly is found in radiating crystal clusters similar to those found on this specimen. The ball-like clusters of spike shaped strontianite crystals in this piece wrap around a core fragment of sphalerite. There are dog-tooth calcite crystals near the base also. The rear of the piece (photo 3) appears to have some minor contact points that could be natural or post mining. It measures 2 x 1 5/8 x 1 1/4 inches or 50 x 50 x 32 mm.