Gems, or precious stones, as they are frequently called, are, for the most part, transparent, and have a vitreous or glassy appearance. Their different colours are occasioned by metallic oxides ([21]) of various kinds, with which they are impregnated. Some writers have classed them by their colours, but this is a very uncertain mode, as different gems have not unfrequently the same colour; and, in many cases, the same gems are of different colours. The usual distinction of gems into Oriental and Occidental is also liable to error, as the best gems, from whatever part of the world they are brought, are always called Oriental. The most estimable of all the kinds are the diamond ([50]), ruby ([54]), emerald ([67]), and sapphire ([53]); and stones a grain in weight, and equal in quality, are valued in the following proportions, at 8l. per carat for diamonds, 4l. for rubies, and 3l. for each of the others. The amethyst ([79]), topaz ([61]), and aqua-marine ([61]), are considered of nearly equal value with each other; and the garnet ([70]) is the cheapest of precious stones.
The ancients engraved upon several kinds of gems; but they appear to have been ignorant of the art of cutting the diamond, the ruby, and the sapphire, which were too hard for them to operate upon. The emerald and the noble opal ([102]) were too highly esteemed as precious stones to have often found their way into the hands of engravers. It has been asserted that the ancients did not use the topaz for engraving; but there is extant a beautiful intaglio, representing an Indian Bacchus, which is said to be a topaz. The garnet was often engraved upon: and there are many master-pieces of the art in calcedony ([91]) and carnelian (93.) Onyx and sardonyx ([92]) were employed for that species of engraving in relief called cameos; and, in many instances, it is pleasing to observe with what dexterity the ancient artists availed themselves of the different colours in the alternate zones to express the different parts and shades of their figures.
Most of the gems may be imitated by artificial preparations of glass, coloured by different metallic substances; and it is not easy, by mere inspection, to distinguish the better kinds of factitious stones from real gems. They are, however, discoverable by a deficiency of lustre, and being so soft as, even in the most perfect kinds, to yield to the point of a steel instrument.
The cutting and polishing of gems is the work of the lapidary, and is in general thus performed:—The shape most proper to be given to any particular gem being determined on, the stone is cemented to the end of a stick, and the different facets are formed by a mill contrived for the purpose. This mill is a plate of copper, or an alloy of lead and tin, to which an horizontal motion is given by very simple machinery, and the surface of which is charged either with diamond powder and oil, or with fine emery and water. A thick peg of wood called a gauge, pierced with small holes in all directions, is set upright on the lapidary’s bench, close to the mill, and the process of shaping the facets thus takes place. The stone is placed on the surface of the mill, the opposite end of the stick to which it is cemented being inserted in one of the holes of the gauge. In this position it is kept steady by the workman, with his right hand, whilst, with the other, he puts the mill in motion. The skill of the lapidary depends on regulating the velocity of the mill, and pressing with more or less force on the stick, with an almost imperceptible tendency to one or other direction in different stages of the work, examining each facet at very short intervals, in order to give as great precision as possible to its size and form. This part of the business being completed, the cutting mill is taken out, and replaced by one of brass, on which the polishing is performed by means of fine emery ([58]), tripoli, and rotten stone ([119]), exactly in the same manner as is practised in the first stage of the process for setting the facets.
DIAMOND.
50. The DIAMOND, or ADAMANT of the ancients, is the most valuable of gems, and the hardest of all known bodies; when pure, it is perfectly transparent.
In a rough state, diamonds have usually either the form of rounded pebbles, with a shining surface, or they are crystallized in the shape of octohedrons, or double four-sided pyramids. ([Pl. II, Fig. 5, 6].) Though for the most part colourless, they are sometimes yellow, green, blue, blackish, or rose-coloured.
The best diamonds are brought from the East Indies. The principal mines are those of Raolconda and Coulour, in the province of Golconda; and that of Soumelpour, or Goual, in Bengal. At Raolconda they are found in the deep crevices of rocks. Persons, by means of long iron rods, with hooks at the end, draw out from these crevices the loose contents, and afterwards wash them in tubs, for the purpose of discovering the diamonds.
The first discovery of diamonds at Coulour was about two centuries ago, by a countryman, who, on digging his ground to sow millet, accidentally found one of these stones of large size. From that period the whole adjacent plain began to be searched to the depth of from ten to fourteen feet; and the work was, at one time, so extensively pursued, that nearly 6,000 persons were employed in it. At Soumelpour the diamonds are found amongst the sand and gravel of the river.
Diamonds are likewise found in the island of Borneo, and in several parts of South America. The mode by which they are obtained from one of the rivers of Brazil has been described by Mr. Mawe. The current is turned, and part of the bed of the river being laid dry, the mud is taken up and washed, by negroes, in places prepared for the purpose, through which a portion only of the stream is allowed to flow. As soon as all the earthy particles have been washed away, the gravel-like matter that remains is raked together, the stones are thrown out, and what diamonds happen to be present are found amongst the refuse that is left.