Why does a green ring appear around hard-boiled eggs

Bonus tips for perfect results
Use eggs that are a little less fresh: an egg that's too fresh is less easy to peel. If it's 5 to 7 days old, it's ideal for this preparation.
Add a pinch of salt or a dash of vinegar to the water: this reduces the risk of cracking if a shell splits during cooking.
Store them in their shells in the refrigerator for up to a week. Once peeled, consume them within 2 to 3 days.
What to do with your hard-boiled eggs?
They're perfect for adding to a salad, filling a sandwich, or even as a quick appetizer (hello deviled eggs!). You can also grate them over avocado toast, chop them into a potato salad, or serve them cut in half with a dollop of homemade mayonnaise.

The secret? Just the Right Timing
No need to overcomplicate things: the key to avoiding the green ring is simply not to overcook your eggs... and to cool them quickly. An easy habit to adopt, for consistently perfect hard-boiled eggs—as pretty as they are delicious.