I've made these tea eggs a few times over the past few years. They're a traditional Chinese snack, or so the internet tells me. The flavoring is mostly from soy sauce, but the spices and tea give the eggs a very complex flavor. I don't remember where I got the recipe; I have it written on a piece of paper and stuck to the fridge.
The marbled surface of the eggs comes from partially hardboiling them, then cracking the shells all over before cooking the rest of the way and and steeping them for a few hours or overnight. You have to crack the eggs pretty well to get the marbling. Here's one example of how mine looked before steeping.
3/4 cups soy sauce
2 star anise
2 tbsp black or oolong tea
1 tsp sugar
1 cinnamon stick
1 tbsp peppercorns
Some tangerine or orange peel
Place eggs in a small saucepan and cover with water. Bring to a boil, lower the heat, and simmer for 3 minutes.
Run eggs under cold water and, when cool enough to touch, crack all over with a spoon. Return to saucepan with still-hot water.
Add soy sauce, star anise, tea, sugar, cinnamon, peppercorns, and citrus peel. Bring to a boil, lower heat to a simmer, and simmer for 40 minutes. Turn off heat and let steep for a few hours or overnight.