To fashion a rough diamond into a brilliant, the first step is to modify the faces of the original octahedron, so that the plane formed by the junction of the two pyramids shall be an exact square, and the axis of the crystal precisely twice the length of one of the sides of the square. The octahedron being thus rec tified, a section is to be made parallel to the common base or girdle, so as to cut off 5-18ths of the whole height from the upper pyramid, and 1-18th from the lower one. The si perior and larger plane thus produced is called the ri tabk, and the inferior and smaller one is called the collet; in this state it is termed a complete square table diamond. To con vert it into a brilliant. two triangular fa cets are placed on each side of the table, thus changing it from a square to an oc tagon ' • a lozenge-shaped facet is also placed at each of the four corners of the table, and another lozenge extending lengthwise along the whole of each side of the original square of the table, which with two triangular facets set on the base of each lozenge, completes the whole number of facets on the table side of the diamond, viz., 8 lozenges and 24 triangles. On the collet side are formed 4 irregular pentagons, alternating with as many irregular lozenges radiating from the collet as a centre, and bordered by 16 triangular facets adjoining the girdle. The brilliant being thus com pleted, is set with the table side upper most, and the collet side implanted in the cavity made to receive the diamond. The brilliant is always three times as thick as the rose diamond. In France,_ the thickness of the brilliant is set off into two unequal portions ; one-third is reserved for the upper part or table of the diamond, and the remaining two thirds for the lower part or collet (o lease). The table has eight planes, and its circumference is out into facets, of which some are triangles, and others lo zenges. The collet is also cut into facets called pavilions. It is of consequence that the pavilions lie in the same ordef as the upper facets, and that they corre spond to each other, so that the symme try be perfect, for otherwise the play of the light would be false.
Although the rose diamond projects bright beams of light in more extensive proportion often than the brilliant, yet the latter shows an incomparably greater play, from the difference of its cutting.
In executing this, there are formed 32 faces of different figures, and inclined at different angles all round the table, on the upper side of the stone. On the col let (culasse) 24 other faces are made round a small table, which converts the culasse into a truncated pyramid. These 24 facets, like the 82 ahrove, are differ ently inclined and present different fi gures. It is essential that the faces of the top and the bottom correspond to gether in sufficiently exact proportions to multiply the reflections and refractions, so as to produce the colors of the pris matic spectrum.
The other precious stones, as well as their artificial imitations, called pastes, are cut in the same fashion as the bril liant; the only difference consists in the matter constituting the wheel plates, and the grinding and polishing powders, as already stated.
In cutting the stones, they are mount ed on the cement-rod, whose stem is set upright in a socket placed in the middle of a sole piece, which receives the stem of the cement-rod. The head of the rod
fills the cup of the sole piece. A melted i alloy of tin and lead is poured into the head of the cement-rod, into the middle of which the stone is immediately plung ed; and whenever the solder has be come solid, a portion of it is pared off from the top of the diamond, to give it a pyramidal form.
There is an instrument employed by the steel polishers for pieces of clock work, and by the manufacturers of watch-glasses for polishing their edges. It consists of a solid oaken table, the top of which is perforated with two holes, one for passing through the pulley and the arbor of a wheel-plate, made either of lead or of hard wood, according to circumstances ; and the other for receiv ing the upper part of the arbor of the large pulley. The upper pulley of the wheel-plate is supported by an iron prop, fixed to the table by two wooden screws. The inferior pivots of the two pieces are supported by screw-sockets, working in an iron screw-nut sunk into a summer-bar. The legs of the table are made longer or shorter, according as the workman chooses to stand or sit at his employment. Emery with oil is used for grinding down, and tin-putty or coleothar for polishing. The workman lays the piece on the flat of the wheel plate with one hand, and presses it down with a lump of cork, while he turns round the handle with the other band.
The Ruby, Oriental Ame thyst, Oriental emerald, and Oriental To paz, are gems next in value and hardness to diamond ; and they all consist of nearly pure alumina or clay, with a mi nute portion of iron as the coloring mat ter. The following analyses show the affinity in composition of the most pre cious bodies with others in little relative estimation.
Salam.starte is a variety which consists of small transparent crystals, generally six-sided prisms, of pale reddish and bluish colors. The corundum of Batta gammana is frequently found in large six-sided prisms : it is commonly of a brown color, whence it is called by the natives curundu galle, cinnamon stone. The hair-brown and reddish-brown crys tals are called adamantine spar. Sap phire and salamstone are chiefly met with in secondary repositories, as in the sand of rivers, &c., accompanied by crys tals and grains of octahedral iron-ore and of several species of gems. Corun dum is found in imbedded crystals in a rock, consisting of idianite. Adaman tine spar occurs in a sort of granite.
The finest varieties of sapphire come from Pegu, where they occur in the Ca pclan mountains near Syria. Some have been found also at Hohenstein in Saxony, Bilin in Bohemia, Puy in France, and in several other countries. The red va riety, the ruby, is most highly valued. Its color is between a bright scarlet and crimson. A perfect ruby above 81 ca rats is more valuable than a diamond of the same weight. If it weigh 1 carat, it is worth $52; 2 carats, $208; 3 carats, E780; 6 carats, $52000. A deep colored ruby, exceeding 20 carats in weight, is generally called a carbuncle ; of which 108 were said to be in the throne of the Great Mogul, weighing from 100 to 200 carats each : but this statement is proba bly incorrect. The largest oriental ruby known to be in the world was brought from China to Prince Gargarin, governor of Siberia. It came afterwards into the possession of Prince Menzikoff, and con stitutes now a jewel in the imperial crown of Russia.