3rd Gen sevii island problem

So i've beaten the elite four and obtained national pokedex (60 pokemon), but the cave on island 1 behind the two rockets still has not opened up. I'm stuck, any help?

I'm sorry, but my memory is a little foggy.
Are you sure that this is on One Island?

yeah right before you go up to moltres, there are two rockets talking about the right place to dig. I looked it up a while ago and it showed a picture of the cave opened on serebii, so i believe this is the place the ruby or saphire stone is.

Have you spoken to Celio again after you defeated the Elite Four?
I think that's what trigger's them to battle you.
Also, yes, this is where you get the Ruby.

It should have opened up once you've beat the Elite Four. So uh... I'm clueless. XD

But if you want the Sapphire, you'd have to go to Six Island's cave in the middle of the field part. When you get to the bottom of the cave, you'll see the stone. A Team Rocket Professor comes and takes it first. Chase him back to Five Island where the Rocket HQ is, and get through the battles and mazes. After that, retrieve the Sapphire from the boss.

You've got to talk to Celio and actually get the quest for the Ruby & Sapphire before the cave opens up. So just go talk to him and accept the favor he asks you to do for him.