International Rock Day is celebrated on June 13. Why do rocks need a day of recognition? Because these hardy heroes have played a big part in human history since the Stone Age. They served as humanity's first tools and as nature's journals, carrying fossils of ancient organisms and documenting changes in geological processes. A great example of the latter is Zhangye National Geopark’s Rainbow Mountains in China. This geological wonder displays vivid colours: reds, oranges, yellows, and purples, which are layers of sedimentary sandstone rock. These unique mountains were formed through erosion over millions of years, creating the patterns and colours that define this landscape. No matter what your favourite category of rock is—metamorphic, sedimentary, or igneous—rocks of every kind helped build the world we live in today.