Do it by all means. My dad died in 1987 and when Mother died in 2004, she still could not decide on a headstone. They have one now. When we went camping, Mother had collected river rocks (one a year) for a fireplace she never had built. We used the river rock for a "fireplace" headstone and the oval where the fire would be is a black granite marker for the family name. We have no problem telling anyone how to find the plot.