The Chemistry, Properties and Tests of Precious Stones

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rm in the divided surface of each piece severed. Thus we find a piece of wood may be "broken" or "chopped" when fractured across the grain, no two fractured edges being alike; but, strictly speaking, we only "cleave" wood when we "split" it with the grain, or, in scientific language, along the line of cleavage, and then we find many pieces with their divided surfaces identical. So that when wood is "broken," or "chopped," we obtain pieces of any width o

reak being at any angle and not necessarily parallel to its bed. A striking example of this is seen in slate, which may be split in plates, or lamin?, with great facility, though this property is the result of the pressure to which the rock has been for ages subjected, which has caused a change in the molecules, rather tha

cleavage will take place much more readily than on the others, these planes or lines also showing different properties and angular characters, which, no matter how much fractured, always remain the same. It is this "cleava

ucture are familiar, and it is then placed in a test-tube and gently heated, cleavage will at once be evident. With a little crackling, the chlorate splits itself into many crystals along its

es evident, it is possible easily to reproduce the same form over and over again by splitting, whereas by simply breaking, the form of the crystal would be lost; just as a rhomb of Iceland spar might be sawn or broken across the middle and its form lost, although this would r

s crystal was almost beyond computation, but it had a flaw in the centre, and in order to cut out this flaw it was necessary to divide the stone into two pieces. The planes of cleavage were worked out, the diamond was sawn a little, when the operator, acknowledged to be the greatest living expert, inserted a knife in the saw-mark, and with the second blow of a steel rod, the marvellous stone parted precisely as intended, cutting the flaw exactly in two, leaving half of it on the outside of each d

ces of which are now called "The Stars of Africa," together with a model

cularly is this seen in the diamond "brilliant," which plainly evidences the four cleavage planes. These cleavage planes and their number are a simple means of identification of precious sto

y when the stones are chemically coloured to imitate a more valuable stone. In such cases the cleavage of one stone is often of paramount im

of crystals, but without a crystalline structure. It is then called a pseudomorph, which is a term applied to any mineral which, instead of having the form it should possess, shows the form of something which has altered its structure completely, and then disappeared. For instance: very often, in a certain cavity, fluorspar has existed originally, but, through some chemical means, has been slowly changed to quar

come coated or "invested" by another foreign substance, which thus takes its shape; or actual chemical change takes place by means of an incoming substance which slowly alters the original substance, so that eventually each is fal

, instead of having sharp, well-defined angles and edges; their surfaces also are not good. These stones are of little value,

carbonate of lime. Such a condition is called dimorphism; those minerals which crystallise in three systems are said to be trimorphous. Those in a number of systems are polymorphous, and of these sulphur may be taken as an example, since it possesses thirty or

lness of the stone to a remarkable extent, and at the same time fo