Specimens of sapphire corundum from Madagascar. Corundum is an aluminium oxide mineral and contains traces of iron, titanium and chromium which influence the crystals colour. With a hardness of 9 on the Moh's scale of hardness, corundum is used extensively in industry as an abrasive as it can scratch almost any other mineral except diamond. Crystals vary in size from roughly 0.5 - 1 cm in length and are available as single crystals or by weight. Samples are supplied in a polythene bag with an information label and are a must have for any crystal or mineral collection, they would also be ideal for educational use when students are learning about the Moh's scale as well as assisting students to understand crystals systems and forms.