Corundum is a sesquioxide of aluminum, with the percentages aluminum 53.2, oxygen 46.8. Its hardness is 9 in the scale of which diamond is 10, and no other mineral except the latter equals it in hardness. This hardness gives it a wearing quality as a gem second only to the diamond. The varieties of Corundum differ slightly in hardness, sapphire being the hardest. Noble Corundum has a brilliant, vitreous luster, which, while not equal to that of the diamond, is superior to that exhibited by almost any other gem. Corundum is a heavy mineral, its specific gravity being four times that of water. This high specific gravity affords an easy means of distinguishing the gems of Corundum from those of other species. Corundum is infusible and is not attacked by acids. It crystallizes in the rhombohedral division of the hexagonal system, certain crystal forms being characteristic of the two varieties, ruby and sapphire. Thus ruby tends to crystallize in flat rhombohedral crystals, while sapphire generally forms in longer, hexagonal prisms. (See colored plate in November number.) Corundum is doubly refracting and dichroic. Of the different colors of Corundum above referred to, the blue or sapphire is most common, the red or ruby next. The other colors occur rather sparingly, green having been almost unknown until the discovery of the Montana sapphires. The nature of the coloring ingredient of the different varieties of Corundum is not known, but there is some reason for believing it to be chromium, for Fremy obtained artificial red and blue Corundum by mixing chromium with his other ingredients, after many attempts to obtain the desired color had failed. Red Corundum varies in hue from rose to deep red. That of the latter tint is the true ruby, the color known as pigeon’s blood being most highly prized. Faultless stones of this color have long been the most valuable of gems, exceeding the diamond in price, weight for weight, unless the latter is colored. Rubies above three carats in weight are about ten times more valuable than ordinary diamonds of the corresponding weights. But few rubies exceeding ten carats are known. The King of Pegu is reported to have one the size of a hen’s egg, but as no one has ever seen it the story may well be doubted. In the crown of the Empress Catherine was, however, one the size of a pigeon’s egg. There is also a large uncut ruby in the British crown, which Ruskin calls the loveliest precious stone of which he has any knowledge.