To execute this installation, route rough-in hot and cold plumbing lines to a low wall location eighteen inches above the finished floor, installing a mixer valve and a long-reach wall spigot. Build a reinforced concrete or cement-board support plinth on the floor, securing it to the subfloor with structural screws and waterproofing the base perimeter. Set the carved river rock boulder onto the plinth, using flexible masonry shims to level the upper polished rim before securing the base with heavy-duty construction adhesive. Install a grid drain assembly into the pre-drilled drain hole of the rock basin, connecting the tailpiece to a dedicated low-profile P-trap inside the wall or floor cavity. Seal the polished interior stone bowl with two coats of food-grade stone impregnator to prevent mineral deposits from standing water. Complete the installation by surrounding the boulder base with loose, smooth basalt pebbles to create a tranquil Japanese garden look.