Nickel doesn't directly turn carbonate green. Instead, it's the formation of nickel carbonate (NiCO₃) that creates the green color.

Here's how it works:

1. Nickel ions (Ni²⁺) react with carbonate ions (CO₃²⁻): This happens when a nickel-containing substance is exposed to an environment rich in carbonates, such as water containing dissolved carbonates or air containing carbon dioxide.

2. Nickel carbonate (NiCO₃) is formed: The reaction between nickel ions and carbonate ions produces nickel carbonate, a solid compound.

3. Nickel carbonate is green: Nickel carbonate has a characteristic green color. This is due to the way nickel ions absorb and reflect light, resulting in the green hue.

So, it's not the nickel itself that turns green, but rather the formation of nickel carbonate that leads to the green color.