Borders, boundary markers, and entrance markers can be quite interesting places (and often places for geocaches!)
The boundary stones marking the original borders of the District of Columbia are well known. Many of them still survive to this day.
However there is a entrance marker (boundary stone?) located in Blair Circle that is very unique. I couldn't find much about this stone other than a short blurb on Wikipedia. This marker is between the North Corner boundary marker and the former site of the Northeast No. 1 boundary marker of the original District of Columbia, each of which have caches associated with them.
The stone is easy to miss, especially during the summer since it is obstructed by greenery in the circle. It is very difficult to see driving by, but it may be seen on foot. I have included a waypoint of where the marker is, but I do not recommend going into the circle to get close to it since this is a very busy traffic circle and there isn't a crosswalk to get into the center circle. From the image I have seen on Wikipedia (below) it does have a distinctive "MD" for Maryland on the part that is sometimes visible. It can be viewed from the pedestrian island separating the lanes of traffic of 16th St on the Maryland side.
Image Source: Google Maps in November 2016
Image Source: Wikipedia
This cache is a simple hide located on the MD side of the border.
I parked on N. Portal Drive NW while hiding this cache, but pay attention to the parking signs depending on the time of day. Also use extreme caution using the crosswalks to get around the circle.