The drops, unless the finished stone is required to have a similar pear shape, are divided longitudinally through the central core into halves, which in both shape and orientation are admirably suited to the purposes of cutting; as a general rule, the drop splits during cooling into the desired direction of its own accord. [Illustration: FIG. 55.—Bubbles and Curved Striæ in Manufactured Ruby.] Each drop is a single crystalline individual, and not, as might have been anticipated, an alumina glass or an irregular aggregation of crystalline fragments, and, if the drop has cooled properly, the crystallographic axis is parallel to the core of the pear. The cut stone will therefore have not only the density and hardness, but also all the optical characters—refractivity, double refraction, dichroism, etc.—pertaining to the natural species, and will obey precisely the same tests with the refractometer and the dichroscope. Were it not for certain imperfections it would be impossible to distinguish between the stones formed in Nature’s vast workshop and those produced within the confines of a laboratory. The artificial stones, however, are rarely, if ever, free from minute air-bubbles (Fig. 55), which can easily be seen with an ordinary lens. Their spherical shape differentiates them from the plane-sided cavities not infrequently visible in a natural stone (Fig. 56). Moreover, the colouring matter varies slightly, but imperceptibly, in successive shells, and consequently in the finished stone a careful eye can discern the curved striations (Fig. 55) corresponding in shape to the original shell. In a natural stone, on the other hand, although zones of different colours or varying shades are not uncommon, the resulting striations are straight (Fig. 56), corresponding to the plane faces of the original crystal form. By sacrificing material it might be possible to cut a small stone free from bubbles, but the curved striations would always be present to betray its origin. [Illustration: FIG. 56.—Markings in Natural Ruby.] The success that attended the manufacture of ruby encouraged efforts to impart other tints to crystallized alumina. By reducing the percentage amount of chromic oxide, pink stones were turned out, in colour not unlike those Brazilian topazes, the original hue of which has been altered by the application of heat. These artificial stones have therefore been called ‘scientific topaz’; of course, quite wrongly, since topaz, which is properly a fluo-silicate of aluminium, is quite a different substance.