When you cmplete a level with 50 rings a giant ring will appear behind the end sign. If you jump into the giant ring you will advance to a bonus level. A Chaos Emerald will be at the end of ALL bonus levels. Only one emerald can be collected from any stage, after which the giant ring will stop appearing.
To do this trick play the Casino Street Zone and run through the level activating the 'slot machines' to gain extra lives. When you have collected the desired number of lives pause gameplay and leave the level to save those lives. Restart the level and repeat the process to get as many lives as you want.