**The egg is overcooked.**
It is completely **safe to eat**, but the color change means the egg was subjected to too much heat or was not cooled quickly enough.
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### What Causes the Green Ring?
When an egg heats up for too long, the **sulfur** in the egg white reacts with the **iron** in the egg yolk. This chemical reaction forms **ferrous sulfide** (iron sulfide) on the surface of the yolk, which gives it a greenish-gray tint and a slightly sulfurous odor or chalky texture.
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### How to Avoid It Next Time
1. **Watch your cook time:** Hard-boiled eggs generally need **9 to 12 minutes** in gently simmering water.
2. **Use an ice bath:** Immediately transfer the eggs from the hot water into a bowl filled with ice water for 5 to 10 minutes. This stops the residual cooking process instantly.
