Copper mine stalactites. Stalactites hanging from the ceiling of a cavern in a copper mine. Like the equivalent in limestone rocks, these stalactites form as minerals are dissolved out of the rock by water sinking through the ground. The minerals are then deposited on outcrops on the ceiling as the water emerges from the rock and comes into contact with the air. The water drips to the floor below, and long icicle-like formations of copper minerals are left behind. It can take thousands of years for such stalactites to form.