Part of a mechanical watch movement, coloured scanning electron micrograph (SEM). The image shows some of the jewels used as bearings in a wristwatch. Natural gem bearings, mostly garnets, rubies and sapphires, were invented in the early 18th Century. At the turn of the 20th Century a method for producing synthetic sapphires was invented and now most jewel bearings are synthetic sapphires made from crystalline aluminium oxide known as corundum. This is an extremely hard substance that can be highly polished thereby reducing friction. Red sapphires known as rubies are produced by introducing impurities, all other colours are known as sapphires but there is no difference in their properties as bearings. The image also shows the balance wheel with the steel balance spring (centre image). These regulate the watch to help keep accurate time. Magnification x13 at an image size of 10 cm wide.