More info on backsplashes via This Old House... (edited by me)
Stone tile is set much like ceramic tile: on a mortar bed or cement backerboard with thinset adhesive, or over drywall with mastic. But, says O'Rear, setting highly reflective stone tile is less forgiving than ceramic tile because uneven surfaces are glaringly obvious. Some stone is uneven in thickness, which means it should be set in mortar instead of thinset. A medium mortar bed of up to 3/4-in. thickness can be applied without causing shrink cracks or damaging tile as it cures. For dense, nonporous stone, O'Rear says, the installer should use a high-quality polymer-modified thinset to improve adhesion.
You can apply backsplash tile over a mortar bed, with thinset adhesive on cement backerboard or directly to drywall with a premixed adhesive called mastic. This last approach is not the best idea for shower stalls or tub surrounds where lots of water is splashed on the walls. But it's generally fine for a backsplash in the kitchen as long as you seal the seam between backsplash and counter.
If you are adding a new counter, put it in before the backsplash. That makes for a more watertight installation, and the backsplash hides the gap between the countertop and the wall.
Spread the mastic with a notched trowel. For standard ceramic tile, a trowel with notches 3/16 in. deep is a good choice...Press the tiles into the mastic and rotate them slightly to spread the adhesive evenly. Although these tiles are butted tightly together, you typically will use plastic spacers between tiles so that the grout lines are even. In most cases, you will have to cut the top row of tiles to fit.
The counter will support the bottom row of tiles as they are set in place. [Behind]the range, the tile had nothing to rest on as the mastic cured. Usually this isn't a problem for standard ceramic tile—the mastic will keep them in place—but for heavier tile, additional support is a good idea. Here, a few small screws were enough to keep the tiles from sagging until the mastic grabbed. Alternately, a wood batten or length of aluminum angle will also work.
Apply grout after the mastic has had a day to cure; use sanded grout for gaps of 1/8 in. or more and unsanded grout for narrower gaps. For a more durable job, choose a grout that is fortified with polymers. The one spot where caulk, not grout, should be used is the seam between the countertop and the backsplash. The right material for that is a top-quality silicone caulk. The Tile Council of America suggests a 1/8-in.-wide caulk joint at the seam. Grout, like many types of stone, should be sealed to prevent stains and water infiltration.
9 hours ago