Good advice by mcp77....Also you might need to incorporate expasion joints into the tiled floor at intervals of between 5 and 8 mtrs depending on what your longest run of tiles is....and what the shape of the room is......and if expansion joints are in substrate then yours need to go where they are.........
lithofin stainstop is good for natural stones of that type.........
floor also might want levelling to achieve a nice flat finish....
Smoothing and Levelling Compounds from ARDEX UK Lithofin stone & tile care - buy online at Leafcutter Design