Sand the top of the desk using medium-grit sandpaper to remove roughness, create an even surface and to eliminate splintering.
Vacuum the desk top to eliminate sawdust and debris.
Spread a 1-inch-thick layer of fine grout over the entire desk top using a trowel, then press rock tiles halfway into the grout. Space each of the rock tiles out by 1/4 inch.
Scrape the trowel over the surface of the rock inlay, which will spread the excess grout into areas with too little grout.
Wipe off the surface of each rock tile using a damp washcloth. Allow the rock inlay top of the desk to dry for 24 hours.
Paint a layer of stone sealant over the desk top using a 2-inch flat paintbrush. Allow the sealant to dry for eight hours.