It appears Nokia is still developing but unless the major carriers get on board there may not be a market in the U.S. of A.
It's believed to be a hoax, PSA: Alleged 'Lumia Emerald' sightings are most likely a hoax | Windows Phone Central
As for the Lumia Emerald, the update is likely to be released during January-February 2015, reports NokiaPowerUser, when the Windows 10 Developer Preview rolls out for Windows Phone (Developer Preview devices).
Also I highly doubt a preview for developers of Windows 10 for Phone will be release around that time frame either. Plus firmware isn't released until there is an actual OS update/release. Windows 10 isn't due till mid next year so Emerald and Windows 10 no way.
Lastly if Emerald is the next firmware after Denim it will probably be with Update 3 of Windows Phone 8.1