Aberdeen's architecture is known for its principal use during the Victorian era of granite, which has led to its local nickname of the Granite City or more romantically the less commonly used name the Silver City because the Mica in the stone sparkles in the sun. The hard grey stone is one of the most durable materials available and helps to explain why the city's buildings look brand-new when they have been newly cleaned and the cement has been pointed. Unlike other Scottish cities where sandstone has been used, the buildings are not weathering and need very little structural maintenance on their masonry. The buildings can however become noticeably darker as a result of pollution and grime accumulated over the years. There has however been great success in cleaning the buildings which can result in their façade being restored back to much how they looked originally.