Very important to seal the tops, only the tops, before you even think of grouting. like, do it now. impregnating sealer. do not let it drip down onto the sides.

Mapei FlexColorCQ grout. Warm Gray. it's premixed and ready to use. grout very small sections and wipe w/very little water. do not over dilute while wiping. make sure you clean well before moving on. (are u doing the grouting? if not, make sure your tile guy knows how to use this stuff). it's a stain proof acrylic grout. not epoxy.

do not use a cement based grout for those rocks. (like sanded, Custom grout you get at HD). The Mapei CQ you can find at lowes, in a tub. Watch the video of how to if u r doing it yourself.

Take a closer look at the flexcolor CQ datasheet, it refrences pebble/river rock.........id contact them.- best.