The Cie'th Stones that come up after mission 7 are not mission markers, so mission 8 is not there. It's by the base camp where you started chapter 11. The crystal globe thing is also just an environmental prop.

As to your secondary question, Cie'th Stones stay on the map so you can find them if you want to retry a misison. Stones that you have found, but have not completed the mission for will flash on the map.

This is obviously too late by 3 years, but I'll add my answer anyway in case someone else with the same question ends up here.

You probably have missed mission 5, which is the requirement to unlock mission 8. Look at the mission list available at the map menu to check whether mission 5 has been completed or not.