Guess what - THERE ISN'T ONE! Silly us! The emeralds are located in different parts of the stages. Thanks to
Philip Buccianti for this guide to the Chaos Emeralds:
Gem 1 (BLUE): Begin under ground zone act 2 as normal, but make sure
you take the upper route near the beginning to where the mine cart is
waiting. Take it and jump off it to the right just after it flies off
the track. If the jump was good, you should land on a high hill with a
mouse-badnik on it. Run up the hill then roll down it to gain some
speed. If you did it right, you will fly to another high ledge with
another mine cart on it. Take it and as before, jump to the right just
after it leaves the track. Climb up the steps you land on and the
emerald is yours.
Gem 2 (YELLOW): Play through the first part of sky high zone act 2,
keeping to the upper ledges until you land on a cloud full of rings.
drop down at the right of this cloud to another one below. nearby is
one of those bouncy clouds with rings. ignore it and jump onto the next
bouncy cloud (WATCH OUT: there is no flooring directly under it!). Use
this to bounce as far right and high as possible onto a very high cloud
with the emerald visible if you jump. Go back to the left of this cloud
and carefully jump towards the left. If you're lucky you should
land on an UNMARKED bouncy cloud which you can use to get the gem.
Gem 3 (PURPLE): Easy. Near the end of aqua lake zone act 2 there is
another zipper tube like the one at the beginning of the act. Enter it
and hold left.
Gem 4 (GREY): Make your way through the second act of green hills until
(quite far into the level) you come across a little platform with two
springs side-by-side near the bottom of a slope. You know you've got
the
right one if there are spikes all about and around 20 rings above the
springs. Anyway, hit these springs at an angle and you should fly until
you land on another platform with two springs side-by-side like before.
Bounce of these and hopefully yoe'll land on a ledge with the emerald.
Gem 5 (RED): Right at the beginning of gimmick mt. zone act 2 is a sea
of spikes with conveyor belts and moving platforms to get you across to
a very steep slope. Near the top of this slope jump to the left into
the
wall and yoe'll be in a secret room. Climb through this room,
collecting
the two extra lives on the way (optional) and at the top to the right
is
the chaos emerald.
Gem 6 (GREEN): In act 3 of scrambled egg zone, if you defeat silver
sonic with all of the other five emeralds, he will give it to you as he
dies.