Must complete all B ranked missions on Gran Pulse.
Here's a link to a guide that shows the locations of all mission and their marks:
http://www.gamesradar.com/f/final-fantasy-xiii-cieth-stone-m...Here's a link to a guide that has suggestions on how to 5-star all 64 missions:
http://www.ps3trophies.org/forum/final-fantasy-xiii/43019-5-...