The green coloration that develops on ancient copper coins is primarily due to the chemical process known as patination or corrosion, which occurs when copper reacts with various elements in the environment. This process is a form of oxidation, where copper metal reacts with oxygen in the air and other environmental factors over time, leading to the formation of copper salts, particularly copper carbonate and copper hydroxide, which give the coins their characteristic green hue. The specific green color is often a result of the formation of copper(II) carbonate (CuCO₃), copper(II) hydroxide (Cu(OH)₂), and copper(II) chloride (CuCl₂), especially in regions with high humidity or exposure to salty air, such as coastal areas.

The patina formation process begins when copper metal is exposed to air and moisture, which allows the metal to react with the oxygen in the atmosphere. The presence of water can accelerate this process, especially if the water contains dissolved carbon dioxide (CO₂), which leads to the formation of copper(II) carbonate. This compound appears as a greenish-blue coating on the surface of the coin. In some environments, especially in coastal or polluted areas, the exposure to chlorine from sea air or industrial pollution can result in the formation of copper chloride, which also contributes to the greenish tint.