Why do we observe a greenish ring around the yolk of hard-boiled eggs

Good news: it’s neither dangerous nor a sign that the egg is spoiled. It’s simply the mark of overcooking!
Perfect cooking: the foolproof method
There is a simple technique, approved by all those who want perfect hard-boiled eggs, without unwanted coloring.
Always start with cold water
Place your eggs in a saucepan, cover them with a few centimeters of cold water and heat gently.

Once boiling, stop everything.
As soon as the water starts to boil vigorously, turn off the heat. This prevents the egg from overheating.Let it rest
Cover the saucepan and leave the eggs in the hot water:

• 9 minutes for a soft and creamy yolk,
• 12 minutes for a firm yolk, ideal for a picnic.