Ruby (Corundum)

Ruby is one of the varieties of corundum. It crystallises in bipyramidal, prismatic, tabular or rhombohedral forms. It takes on various shades of colour, but in the specific case of ruby it is always red in tone. Heavy and insoluble in acids, it does not melt and can be scratched only by diamond. It is sometimes fluorescent under ultraviolet rays. It forms in igneous and metamorphic rocks and more rarely in environments formed by the consolidation of deep magmas.

Thanks to its considerable hardness, corundum is used as an abrasive. It is also used in the construction of precision instruments, although the clearest and most colourful varieties have a purely gemmological use.