Green Hill Zone (8-bit)
From Sonic Retro
Green Hill was the first zone on Sonic the Hedgehog for Game Gear and Sega Master System.
It is loosely based on Green Hill Zone from the Sega Genesis version of the game, which was the original.
Difference
The most noticeable difference would be that the colours are less dynamic, and are brighter somewhat. The same enemies are featured, Crabmeat and Buzz Bomber.
Chaos Emerald
Green Hill's chaos emerald is found in Act 2, on a ledge which is reached by descending a waterfall.