You unlock SS8 when you get an A rank or better in all other special stages.
The "chaos emerald" in SS8 is a bullshit emerald designed by JTE to let you actually beat SS8, because apparently hitting the "goal" sign in special stages does nothing if you don't actually have an emerald. Earlier builds allowed you to hit "goal" but because of coop related stuff, that was nixed. So the 8th emerald is filler