Without realising it I have assembled, over time, a small collection of rocks in my office, each picked up from places visited. It started, I suppose, with a piece of quartz from Chek Lap Kok island off Hong Kong, retrieved as a souvenir before the island was transformed into a major international airport in the early 1990s. It is a reminder of the project I led to design the identity and sign system for the new Lantau and Airport Railway, which was built to link the airport to Lantau, Kowloon and Hong Kong island. Over the years this stone has been joined by others: pink granite from Brittany, flint from the South Downs, gneiss and schist from Cornwall and the west coast of Scotland. The forms, textures and patterns are endlessly varied; they have a totemic quality, these small mementos of landscapes tracked and travelled. In their own place rocks often play host to lichen which add layers of colour and texture, the interplay of vegetable and mineral creating its own intricate designs.