The Oriental ruby, sapphire, emerald; chrysolite, topaz and amethyst are all varieties of corundum, a mineral which is only inferior to the diamond for hard¬ness, and is found in crystalline limestone (e.g. in Burma), and in alluvial sands and gravels derived from igneous and metamorphic rocks.

When coloured, as in the case of all the varieties mentioned above, it shows a striking dichroism, being deeply coloured when viewed along the direction of the vertical axis and pale coloured when viewed at right angles to this direction. Hence the red variety, the Oriental ruby, can be distinguished by this experiment from the garnet, the Balas ruby (found in Bokhara) and the Spinel, which are cheaper, more common and more brittle, and are not corundum gems.

f the different varieties of corundum mentioned above the ruby is red, the sapphire blue, the emerald green, the chrysolite yellowish-green, the topaz yellow and the amethyst violet. The.finest collection of topaz  is said to be that in the National History Museum at  South Kensington.