But when the stone has been polished and set in such a manner that the test of hardness cannot be easily applied, then the easiest method of distinguishing its character is to expose it to the rays of artificial light and observe their effects. If the stone, when placed at a certain distance from the candle, flashes out the prismatic hues in a marked manner, then the observer may safely conclude that the object is either a diamond or one of the artificial gems known as strass. To distinguish them by means of the eye alone will be a difficult task, for both possess very high refractive powers and exhibit remarkable displays of the prismatic hues; but the application of a fragment of diamond, sapphire, or topaz to the object will at once make known its character, and even the test of the steel file will betray the softness of the glass. The transparent zircons closely approach the diamond in brilliancy, and are among the rarest of gems; in fact, a fine specimen of perfect purity would be worth more to the amateur than a diamond of equal weight. The pure yellow are declared by King to be among the rarest and most beautiful of gems on account of their remarkable brilliancy and lustre. He also states that they are seldom found above two karats in weight. The white zircon, which resembles the diamond so closely, is always minute and rarely above one karat in size. All that are known in commerce come from Matura, in Ceylon, and are sometimes called Matura diamonds. In times past they were in great demand for incrustation work of watches and jewelry, for when cut in the form of roses and set upon brilliant foil they could not be easily detected by the eye from the diamond. At the present day they are seldom seen in commerce, but whether from scarcity, or from an improvement in the ideas of honor in the jewellers, we are unable to state. We know, however, that they are still mined by the natives of Ceylon, and are used in Asia for ornamental purposes. The white spinel is sometimes mistaken for the adamas; but its inferiority in hardness and its want of the prismatic hues render its detection easy. The white topaz, when of two or three karats weight, resembles the diamond in the rough very closely, especially when in the modified or globular forms of crystallization. The degree of specific gravity is exactly the same; but the test of hardness and the want of the prismatic display serve to dispel all doubts as to its nature. Neither of these gems, nor the white sapphire, has that peculiar sharp flash of the diamond which is known as the adamantine eclat. It has been stated by Hoppè that some of the Brazilian diamonds do not exhibit any more play of color when cut than rock crystal; but we have no doubt but that the writer is in error, and has probably been deceived by some of the white spinels or topazes which are sometimes found in the diamond mines of Brazil and are of great brilliancy.