Portland Blue Limestone
From the B.R.E. Stone List:
The stone is a dense limestone from the Purbeck limestone beds that form the upper part of the Portlandian formation ( Jurassic) on the Isle of Portland, Dorset.
The stone is supplied by Albion Stone quarries.
The word blue is a mild misnomer for a description of the colour of the stone; the colour is closer to a pale brown and a rich grey. The grey and the brown intermingle with each other.
The Portland Blue is considerably harder than the Portland Whitbed and takes a polish.