Sapphire. Coloured scanning electron micrograph of a fractures sapphire. Sapphire is aluminium oxide ( corundum) in the purest form with no porosity or grain boundaries, making it theoretically dense. Trace amounts of iron and titanium can develop a blue color in corundum. Blue corundums are known as "sapphires." The name "sapphire" is used for corundums that range from light blue to dark blue in color.The glass-like material covering supermarket barcode scanners is often sapphire crystal due to its scratch resistance. The most common consumer application for sapphire crystal is in the creation of extremely hard and scratch-resistant coverings for lenses or high-end watch faces. Magnification: x7000 when printed at 10 centimetres across.