Malachite is a hydrated copper carbonate mineral. It is bright green and frequently botryoidal.
Minerals with a botryoidal habit look bumpy and bubbly. It is tempting to think of this bubbly appearance as being due to heat. It is actually due to the way the mineral grows.
In this case, malachite grows in fluid rich environments within cracks and fissures in the crust. The ingredients of the malachite are dissolved in the fluid. They precipitate out to form the mineral. The first crystals of malachite form little sphericles. It is easier to grow a crystal around an existing crystal than it is to start growing a new one. The mineral continues to grow around these points. It forms larger and larger balls. Each ball grows at roughly the same rate. Over time, the balls grow into each other. This is what gives malachite its bubbly appearance.
Changes in conditions will lead to slightly different chemistry in the precipitated mineral. The resultant malachite will be slightly different shades as it grows around the balls. Slicing through the balls leads to a banded appearance, the material often looks like it is made up of rings and little eye. In China, these bands are supposed to represent the eyes of a peacock feather. The Chinese name for malachite translates as "peacock stone
Malachite is the main ore of copper. Copper can form as a native element, but the first copper materials used by humans seem to have been derived from smelting malachite, not from native copper.