Clean a large flat river rock by scrubbing it with a nylon brush under running water. Let the rock dry thoroughly.
Paint the rock a solid dark color using acrylic paint. Cover the rock with paint by painting one side at a time. Allow the paint to dry completely.
Paint the pet's name and its birth and death dates on the rock using a finely tipped paintbrush and a pale paint color. Allow the paint to dry completely.
Spray the rock with spray varnish. Spray varnish protects the paint from the elements. Allow the varnish to dry completely.
Press the marker into the earth at the grave site, writing side up.