The Chemistry, Properties and Tests of Precious Stones

lline St

o all must be discussed, in order to bring the gems into separate classes, not only because of some chemic

, since their crystalline structure forms a ready means for the classificat

, speaking broadly, that the same substance gives the same crystal, no matter how its character may be altered by colour or other means. Even when mixed with other crystallisable substances, the resulting crystals may partake of the two varieties and become a sort of composite, yet to the physicist they are re

ge maybe that its planes and angles might have been measured and defined by rule and compass. This shows how impossible it is to alter the shape of a crystal. We may dissolve it, pour the solution into any shaped vessel or mould we desire, recrystallise it and obtain a solid sphere, triangle, square, or any other form; it is also possible, in many cases, to squeeze the crystal by pressure into a tablet, or any form we choose, but in each case we have merely altered the arrang

o that in the nine planes of symmetry of the cube we get three axes, each running through to the opposite side of the cube. One will be through the centre of a face to the opposite face; a second will be through the centre of one edge diagonally; the third will be found in a line runnin

to the shape of the crystal, taking certain characteristic forms, such as the square, various forms of triangles, the recta

ny of these groups are analogous, so that on analysing them still further we find that all the known crystals may be classed in six separate systems according to their planes of symmetry, and all stones of the same class, no matter what their variety or complexity may be, show forms of the same

nometric, or regular-there are, as we have seen, thre

ual and in one plane and all at 120° to each other; the fourth axis is not always equal to these three. It may be, and often is,

n this case, though they are all at right angles, two only of them are equal, the third, consequently, unequal. The vertical or principal axis is often much longer o

three axes; but in this case, none of them are equal, though the two lateral axes are at right angl

em unequal. The two lateral axes are at right angles to each other, but the principal or vertical axis, whi

c, or asymmetric-the axes are again three, but in this

cture as the diamond are placed in the same group. Further, when the methods of testing come to be dealt with, it will be seen that these particulars of grouping form a certain means of testing stones and of distinguishing spurious from real. For if a stone is offered as a real gem (the true stone being known to lie in the highest or cubic system), it follows that should examination prove the stone to be in the sixth system, then, no matter how coloured or cut, no matter how perfect the imitation, the test of its crystalline structure st