Sonic The Hedgehog 2

To enter a special stage you need to have collected 50 rings. Once you pass the checkpoint a circle of stars will appear above it - just jump into it to begin the special stage.

Make use of the save feature wherever you feel you need it - at the beginning of the stage and/or after each checkpoint. Once you have the emerald in your grasp you'll be transported back to the main level and you will receive this achievement.

The way you earn the Chaos Emeralds in Sonic The Hedgehog 2 has dramatically changed, as the Special Stage are different. You will also have more work to do as from this game onwards, there are 7 Chaos Emeralds to collect.

To enter a Special Stage, you must first collect a total of 50+ Rings. Then successfully complete find a checkpoint area in a level. It will then start spinning a circle of stars around the top of it. Jump into these stars and you'll be taken to the Special Stage.

In the Special Stage Sonic/Tails will be on a race course and you must successfully collect the required amount of rings the game asks you for. If you succeed, you'll do another lap with a new requirement of rings. Do this a total of 3 times, and you'll win the Chaos Emerald, and the achievement. If you fail you'll be eliminated and return to the checkpoint in the level you last touched.

The Rings in the arenas are usually scattered, as some make a trail, some go on the sides, some go close to enemies and some even are high up the top of the screen. So you'll need eyes all around especially the harder levels.
There are also Bombs which are in the arena as well. They are deadly as if you get hit, it will delay you from collecting anything for a short period of time and you will lose a few rings. So you must watch carefully where you move.

Tip: It is best to earn this achievement with one character. If you play with Sonic & Tails, the AI Tails is 100% useless as he'll only follow you, and will likely get knock out by the bombs or get lost.

Tip: When you get on the harder courses, each time you successfully pass a ring requirement... SAVE!! They get extremely harder and saving is the best way to succeed.

Note: The last level where you'll see checkpoint is on top of the Aircraft in the Wing Fortress Zone. There are no more checkpoints after this area.

Chaos Emerald - 10G (Original Achievement)

Difficulty: 3/10

To get this achievement you need to get 50 Rings on any stage and run past a checkpoint, right afterwards, stars will appear above the checkpoint. (I highly suggest creating a "save state" right here just in case you fail, so you can load up the game and try again.) Now simply jump into the stars to enter the Special Stage and go for the Emerald. Your goal is to collect the set amount of rings the game asks for on screen, before you reach the end. Once you do so, the game will ask you for a second amount of rings. After clearing that, the game will ask for a third and final amount of rings. After getting that amount you will receive your Chaos Emerald and this achievement. (Note: There may be black bombs in the stages so dodge them at all costs or you will lose a nice amount of your rings.