This is a huge, natural malachite
formation from the Congo. It measures 8.3" wide and probably weighs about 25 lbs. It has several malachite "stalactites" protruding from it, a fairly rare occurrence.
Malachite is a basic copper carbonate with the chemical formula Cu2CO3(OH)2. This mineral is known for its intense green color, along with the wide variety of formations and patterns that it's found in. Malachite can grow in botryoidal masses, stalactitic, and reniform formations, typically as a tight cluster of fanning fibrous needles that make up a seemingly solid mass. As layers continue to stack during formation, a banded pattern can sometimes begin to take shape, which explains the rings in all shades of green that are seen on most polished malachite specimens.