Border stone refers to a stone that is used to mark the boundaries of a property or a region. If you are looking for synonyms for border stone, then you can consider using terms such as marker stone, boundary stone, boundary marker, landmark, or corner stone. These synonyms will help you better describe the function and use of the border stone. Additionally, you can also consider using regional or cultural specific terms such as cairn (used in Scotland), mōkihi (used in New Zealand), or obelisk (used in ancient Egypt). Whatever word you choose, it is important to ensure that it accurately reflects the purpose and function of the border stone.