Certain moisture sensitive stones (green colored stones are a big red flag) known as serpentine are prone to curling from the moisture in thinset. They begin to warp and pull away from the thinset before it's established a solid bond. In these cases, setting them with epoxy...or coating the backs of them with epoxy, then setting them after they've cured with modified thinset is common.
If the resin backing on the tile is smooth, Tec (Brand) wants you to use an epoxy to set. But if the backing is real rough, they'll let you use a high dollar modified. Like most of the stuff we do, it changes with manufacturers and have to look into unfamiliar tiles.
