Rock found, turns green with water-- what is it?

Question

My uncle found a rock 18 ft below the ground in black clay and it was smooth and spear-shaped, the point was rounded and the base was sharpened flat.

When put under water, it turns green. The longer it stays under, the greener it gets.

Anyone have any idea what kind of rock this is? Or know who i can ask or where i can research this?

Answer 1

Copper turns green under water and is mostly found green under ground as copper ore. The statue of liberty is green because it is made of copper. However I don't think copper turns green quickly when submerged under water.