St. Isaac’s Cathedral was designed by the French architect Auguste Montferrand and built between 1818 and 1858. The massive dome is covered with 220 pounds of pure gold and is the 4th largest cupola in the world (after Vatican, Florence, St Paul’s in London). The large exterior columns are made of single pieces of red granite. The interior columns are made of malachite and lapis lazuli. The church was designed to accommodate 14,000 standing worshipers but is now only used as a church on major religious occasions.