What is trachyte and how did it come to be the unsung hero of Sydney's building stones? The answer is found in this fascinating story of how a hard stone quarried in the New South Wales Southern Highlands became the city's most important stone after sandstone. The title provides an apt description: trachyte was Sydney's hard rock. Sandstone with all its virtues was the premier building stone of Sydney's early and middle years but trachyte had qualities sandstone lacked and so it perfectly complemented the 'yellowblock' of our heritage buildings. Sydney's hard rock provided what sandstone, with all its beauty, could not provide. This tough, distinctively coloured igneous stone was first quarried at Mount Gibraltar near Bowral in the 1880s and soon began appearing in the kerbs and gutters along the growing city's streets.