The sapphire is a gemstone that is often used in jewellery making because it is particularly popular due to its unique appearance and special properties. All varieties of corundum that are colourless or coloured are called sapphire. Only the ruby, with its red colour, is excluded. On closer inspection, however, only the blue sapphires are considered to be variations worth mentioning for jewellery making. The blue tones of sapphires can range from light blue to deep dark blue, almost black, which can appear differently depending on the incidence of light. As a corundum variety, sapphires belong to the mineral class of oxides and hydroxides and can be both transparent and opaque. They have a glassy luster, which makes them interesting for jewellery production. As a gemstone, a sapphire is particularly valuable when it is intensely blue in colour but not too dark and has been given an elaborate facet cut. Sapphires have been artificially produced for over a hundred years, and all sizes can be achieved. In addition to its use as a gemstone, sapphire also has an esoteric significance. Here it is associated with peace and tranquillity.