For 30 years, David Marshall has been creating jewelry for some of London’s top jewelry houses and special commission designs for private clients—and last month, he opened his first boutique, at 42 Davies Street in London’s posh Mayfair neighborhood, to highlight his own designs. The two-level store features an array of his diamond and gemstone designs on the main level, including new designs using jade, green tourmalines, seed pearls, blue sapphires, and tanzanite. Prices start at $1,600 for classic diamond p…