All stones are more or less brittle, and will be fractured by a sufficiently violent blow, but the irregular surface of a fracture cannot be mistaken for the brilliant flat surface given by a cleavage. The cleavage is by no means induced with equal facility in the species mentioned above. A considerable effort is required to split diamond, but in the case of topaz or kunzite incipient cleavage in the shape of flaws may be started if the stone be merely dropped on to a hard floor. CHAPTER X ELECTRICAL CHARACTERS The definite orientation of the molecular arrangement of crystallized substances leads in many cases to attributes which vary with the direction and are revealed by the electrical properties. If a tourmaline crystal be heated in a gas or alcohol flame it becomes charged with electricity, and, since it is at the same time a bad conductor, static charges of opposite sign appear at the two ends. Topaz shows similar characters, but in a lesser degree. Quartz, if treated in the same way, shows charges of opposite sign on different sides, but the phenomenon may be masked by intimate twinning and consequent overlapping of the contrary areas. The phenomenon may also be seen when the stones are cut. The most convenient method for detecting the existence of the electrical charges is that devised by Kundt. A powder consisting of a mixture of red lead and sulphur is placed in a bellows arrangement and blown through a sieve at one end on to the stone. Owing to the friction the particles become electrified—red lead positively and sulphur negatively—and are attracted by the charges of opposing sign, which will therefore be betrayed by the colour of the dust at the corresponding spot. The powder must be kept dry; otherwise a chemical reaction may occur leading to the formation of lead sulphide, recognizable by its black colour. Bücker has suggested as an alternative the use of sulphur, coloured red with carmine, the negative element, and yellow lycopodium, the positive element. Diamond, topaz, and tourmaline are powerful enough, when electrified by friction with a cloth, to attract fragments of paper, the electrification being positive. Amber develops considerable negative electricity when treated in a similar manner. Diamond is translucent to the Röntgen (X) rays; glass, on the other hand, is opaque to them, and this test distinguishes brilliants from paste imitations. Diamond also, unlike glass, phosphoresces under the influence of radium, a property characterizing also kunzite. It will be seen that the electrical characters, although of considerable interest to the student, are, on account of their limited application and difficulty of test, of little service for the discrimination of gem-stones. PART I—SECTION B THE TECHNOLOGY OF GEM-STONES CHAPTER XI UNIT OF WEIGHT