This will unlock as you collectively keep getting opals, it will require replaying levels back over again. That said if you're working towards the 100% completion achievement it should pop naturally. You DO NOT need to have all 50,000 at once, meaning you can spend your opals on rangs & costumes & it will still pop once you hit 50,000 total. I had about 7,000 in my actual opal bank when it popped for me.