Limestone is highly porous – some types more than others. Coastal Grey (also known by other names such as Wooden White) is one of these stones. It must be installed with a rapid-drying thinset. If regular thinset is used, the tile absorbs the water in the thinset and it can pull all the way through to the surface of the tile making it appear wet from the back. The below job is an example of this. How long this stone will stay wet is anyone’s guess. It could stay wet for up to 6 months and must be completely dry before the tile can be sealed or grouted, making the shower inoperable until then.
Rapid drying thinsets are not only used for commercial installations, such as a kitchen, where time is of the essence. They are also used with certain stones that may wick moisture from a regular thinset. When possible, it is recommended to purchase the installation materials along with the tile, as the manufacturer of each material will provide any guidelines needed for a successful installation. If this is not possible, ask your tile salesperson to notate any important information such as installation tips or recommendations guided by the material's origin & characteristics.