Gem-Stones and Their Distinctive Characters

by George Frederick Herbert Smith

Very few changes have been made in the method when adapted to commercial use. Coal-gas has, however, entirely replaced the costly hydrogen, and the hammer is operated by a cam instead of an electromagnet, while, as may be seen from the view of a gem-stone factory ([Plate XIV]), a number of blowpipes are placed in line so that their cams are worked by the same shaft, a. The fire-clay screen, b, surrounding the flame is for convenience of removal divided into halves longitudinally, and a small hole is left in front for viewing the stone during growth, a red glass screen, c, being provided in front to protect the eyes from the intense glare. Half the fire-clay screen of the blowpipe in the centre of the Plate has been removed to show the arrangement of the interior. The centring and the raising and lowering apparatus, d, have been modified. The process is so simple that one man can attend to a dozen or so of these machines, and it takes only one hour to grow a drop large enough to be cut into a ten-carat stone.

PLATE XIV

BLOWPIPE USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES

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.

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.

Fig. 56.—Markings in Natural Ruby.