Bluestone flagging for sidewalks was a popular choice from Civil War times to the 20th Century contemporaneous with the growth of Brooklyn brownstones and with the home delivery of coal. It is no surprise to see coal holes in flagstone sidewalks. Most holes were fitted with the decorative cast iron covers still seen today, so it is a treat to see that in some instances an older technology was still in use.
The design for this cover is to have none. Only an outline should be seen or noticed at all. I doubt if the cover and the surrounding stone were originally one, but they certainly are from close pieces. As interesting as the original design, is the design of the stone lid as contemporary relic. Although broken, the owners of the lid have carefully restored the missing pieces with no attempt to disguise their work.
Originally a coal-hole cover, now probably secured and non-functional, but decoratively opaque.
This beauty is on Cranberry St. near Columbia Heights in Brooklyn.