Dense blue-grey with a fine grain. The most weather-durable limestone in common use.
Belgian bluestone is a dense, fine-grained grey limestone that behaves more like a granite than like a soft cream limestone. Absorption is under 1%, it is genuinely freeze-thaw stable, and it wears at the pace of the building it is set into.
The colour shifts with finish in a way that surprises people: honed it is a deep blue-grey, sandblasted it goes soft and pale, flamed it turns nearly silver. Ask for samples in the actual specified finish, not just the stone.
It is the correct answer for a northern patio, a driveway apron, or any step tread that will see a century of boots.
Each of these opens the library filtered to everything suitable for that use, so you can see what Bluestone Grey Limestone is competing against.
Post the project and contractors working in your county bid on it — with this stone already specified.