There are 4 Red Cactuar stones that teleport you to the Nomad Camp. These are not mandatory.
There is 1 Yellow Cactuar stone that teleports you to the Clearwater Marshes. This is not mandatory.
There is 1 Green Cactuar stone and 2 invisible Cactuar stones that summon Gigantuars when you touch them. When all 3 have been activated and the Gigantuars killed, THAT is when you unlock the Blue Gate.
"I have a time machine and own Versus 13. Ask me anything." ~ KingWigglyTuff "What's life like in the year 3000?" - Five Pound Note
One is hard to miss amid the spiny rocks or whatever in the center of the map, one is hard to miss out on a strip of land in the middle of the water, and the one it took me forever to find was behind the big stone at the edge of the water closest to the nomad camp.