In the dichroscope the ruby shows two images, one square of a violet red, the second square being a truer and a paler red. It may be subjected to strong heat, when it changes its colour to a sooty or dirty slate, this varying with the locality in which the stone is found, and the manner in which the heat is applied. But as it cools it becomes paler and greener, till it slowly enrichens; the green first becomes broken, then warmer, redder, and finally assumes its original beautiful blood red. This method of heating is sometimes used as a test, but it is a test which often means the complete ruin of a stone which is not genuine. Another characteristic which, in the eyes of the expert, invariably isolates a real from an artificial ruby is its curious mild brilliance, which as yet has not been reproduced by any scientific method in paste or any other material, but perhaps the safest test of all is the crystalline structure, which identical structure appears in no other stone, though it is possible, by heating alumina coloured with oxide of iron and perhaps also a trace of oxide of chromium to a very high temperature for a considerable time, and then cooling very slowly, to obtain a ruby which is nearly the same in its structure as the real gem; its specific gravity and hardness may perhaps be to standard, and when properly cut, its brilliance would deceive all but an expert. And as in some real rubies there are found slight hollows corresponding or analogous to the bubbles found in melted glass, it becomes a matter of great difficulty to distinguish the real from the imitation by such tests as hardness, specific gravity, dichroism, and the like, so that in such a case, short of risking the ruin of the stone, ordinary persons are unable to apply any convincing tests. Therefore, only the expert can decide, by his appreciation of the delicate shade of difference in the light of a true ruby and that of an excellent imitation, and by the distribution of the colour, which--however experienced the chemist may be, or with what care the colouring matter may have been incorporated in the mass--has been found impossible of distribution throughout the body of an artificial stone so perfectly and in the same manner and direction as nature herself distributes it in the genuine. This alone, even in the closest imitations, is clear to the eye of the expert, though not to the untrained eye, unless the stone is palpably spurious. To one who is accustomed to the examination of precious stones, however perfect the imitation, it is but necessary to place it beside or amongst one or more real ones for the false to be almost instantly identified, and that with certainty. _The Sapphire._