Where abouts in the south part of the 6th island is the sapphire and where?

Dotted hole.

Yes it is on Six Island in the Ruin Valley, but you must clear the Rocket Grunts out of the Ice Cave on Four Island first; when you get the Ruin Valley on Six Island there will be a cave in the center of the valley; a door blocking the entrance of the cave will have unreadable language on it this text will read "Cut", have a Pokemon on your team that knows the move Cut and slash the door down. Once inside you will see a hole in the center of the floor, fall in the hole. This will lead you to four different holes in the floor surrounding you there is a pattern you must enter the holes, there is a hole North, South, East and West of you; I will display the order you must go in right now:

1) Okay, first hole you head to the North one.
2) You will fall to the same room as before the second hole you go in is the West side hole.
3) Almost done, the third hole you must drop down from is the hole East of you.
4) Last hole the fourth hole you head to the one South of you.

Once you done all these steps you will drop to a different room which contains the Sapphire. A man who followed you down to the chamber will steal the Sapphire from you, but not without telling you the second password to the Rocket HQ on Five Island. Once at Five Island enter the base beat the Rocket members and take the Sapphire back.

You have to look for the Dotted Hole. The Sapphire is there.

You have to look for the Dotted Hole. The Sapphire is there.