My business sells monuments. I am wondering what the average person would type into the searchbar that would help me determine my keywords and phrases.
"meaningful tombstones" "monumental tombstones" "customizable tombstones" "tombstones for a loved one" etc..
If I were searching, I wouldn't call it a monument, I would call it a memorial. I don't use the term tombstone either, I would call it a gravestone.
On your web site you might consider having several different versions of the same page. Each version can be written with a particular keyword or keyphrase in mind. If you use a tool like Google Analytics you can eventually determine which page gets the best results.