To dream of the tombstone means that you are coming into some phase of your life where things are changing. To get more detailed dream interpretation, please also see the meaning of death.
Dream dictionary: Tombstone dream meanings
Tombstone indicates traumas originated in childhood and which have not been overcome yet.
To have interaction with a headstone or to encounter tombstone or to see a gravestone, when you are dreaming, then such dream has the symbolic significance of forgetfulness. Headstone in the dream indicates aspects of yourself which you need to accept. Maybe you have buried these qualities of yourself which you need to acknowledge. The message on the gravestone also is important. You should consider what is written on it. This message may indicate a statement about conditions of your current life. Also it can be something from your past. Or it is message from the future.