Another source of rubies is their artificial production, after the method discovered by the French chemist Fremy. These are made by heating a mixture of aluminum sesquioxide, carbonate of lime, barium fluoride and potassium chromate in a porous clay crucible to a temperature of 1500 degrees C. and keeping the mixture fluid for eight days. Well-formed, clear crystals up to one-third of a carat in weight are thus produced, which have the hardness and color of native ruby. They are not considered so valuable as gems as the latter, and can be distinguished by the air bubbles which may be seen with a lens. The expense of making them is nearly equal to the value of native rubies, so that their production is likely to be limited. Rubies were known to the ancients, being mentioned in the Bible in Proverbs and Job. The Greeks and Romans ascribed to the ruby the power of giving light in the dark, and the Hindoos describe the abodes of their gods as thus lighted. The ruby was much worn as an amulet, being supposed to protect the wearer against plague, poison and evil spirits. It was also thought that it would turn dark if its owner were in danger and would not regain its color until the peril was over. The ruby is usually cut in the form of the brilliant, like the diamond, but sometimes the step cut is advantageously employed. The stones from India are usually rounded by the native gem cutters and worn in this manner. Blue precious Corundum or sapphire is more abundant than the red or ruby. Like the red the blue color seems to be due to a content of chromium, since in the artificial crystals already mentioned as produced by Fremy, both colors occur at times in the same crystal. The blue color, however, unlike the reds, disappears on heating. Blue Corundum exhibits various shades from light to dark, the color most highly prized being that known as cornflower blue. A good sapphire should also have high luster and a velvety sheen. As already noted, sapphire is somewhat harder than ruby, and it is also somewhat heavier. The Montana sapphires are said to be especially hard. Sapphires have at the present time not over half the value of a ruby of the same size. A price of forty dollars per carat is an average one for a stone of not over ten carats and, as much larger stones are comparatively common, the price does not increase so rapidly as does that of the ruby with an increase in size.