The mortar that I used has thinset as a component. This is whats recommended by the manufacture, and since its cheaper too, it seems silly to use something else. I would make sure your contractor follows the Eldorado installation instructions, as setting the stone is quite a bit different from setting tile. The isntructions can be downloaded at http://www.eldoradostone.com/shared-downloads/Eldorado-Stone-Installation-Procedures.pdf
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