St. John's, Smith Square, London
If there's a type of stone that characterises London, especially the Government estate and major historic buildings including Buckingham Palace and St Paul's, then that stone is Portland Stone. It's a limestone, dating back to the Jurassic Period; some facies (varieties) are very fossil-rich, with Grove Whitbed being rich in oysters, and Roach Stone being famous for it's 'Osses' heads' (Trigoniid bivalves) and Portland screws, a variety of gastropod mollusc. Other facies are freestones, very suited to carving by stonemasons. There's a reason for the popularity of Portland Stone in London, and for its use to rebuild St Paul's after the Great Fire of London in particular - Sir Christopher Wren owned shares in a certain quarry on the Isle of Portland! His use of the stone set a trend, and now, in addition to the buildings I've already cited, think in terms of Regent Street, the Old Bailey, the Bank of England... One of my favourite buildings is ...