"Just to amuse myself, and keep the good people busy, I ordered them to build this City, and my Palace; and they did it all willingly and well," said Oz. "Then I thought, as the country was so green and beautiful, I would call it the Emerald City; and to make the name fit better I put green spectacles on all the people, so that everything they saw was green."

"But isn't everything here green?" asked Dorothy.

"No more than in any other city," replied Oz; "but when you wear green spectacles, why of course everything you see looks green to you. The Emerald City was built a great many years ago, for I was a young man when the balloon brought me here, and I am a very old man now. But my people have worn green glasses on their eyes so long that most of them think it really is an Emerald City." ...

"I think you are a very bad man," said Dorothy.

The capital of Oz in TheWizardOfOz.

But there was an Emerald City in the HollowEarth, as well, ruled by BryAn -- what was its name? ... The cyclopean city of Yaktavia, built by the fabled inhabitants of Lemuria 100 million years before humanity.

Also, when Danny and Reese were being brainwashed by LegalTenderCoxey to kill the WickedWitchOfTheWest, it took place in an emerald green room. Legal explicitly compared his instructions to the two to the Wizard's orders to Dorothy.