Sapphire is of the same material, differing from the ruby only in color. The ruby owes its fine red color to the presence of oxide of chromium; the sapphire its deep blue to either a lower oxide of chromium or to an oxide of titanium.
Crystallized alumina in the different colors receives different trade names, as Oriental emerald for the green; Oriental topaz for the yellow; Oriental amethyst for the purple; while the water-clear, colorless crystal is known as white sapphire.
The process of manufacture of rubies is carried on with the oxyhydrogen blow-pipe, to whose intense heat the powdered alumina with its coloring oxides is subjected. Rubies have been thus produced weighing twelve to fifteen carats when cut. The average weight of the native Burmese ruby is about one-eighth of a carat. The sapphire and the so-called Oriental stones are prepared in the same manner, with the addition of proper coloring matter.
The emerald and opal have not emerged from the experimental stage, although Becquerel, a French chemist, is reported to have produced opals from solutions of silicates with high-tension electric currents.
To be distinguished from synthetic gems are reconstructed stones, which (as yet only done with the ruby) are pieces of the natural stone fused together. They are very brittle.
The pearl is not produced synthetically, but many imitations exist. The Japanese produce them by fastening a piece of mother-of-pearl in the shells of the pearl-oyster and allowing it to remain there for a number of years.
The turquoise, a phosphate of aluminum colored with copper, is not synthetically produced, although various experiments with its manufacture have been made.
Reproduced by permission of The Philadelphia Museum.