The polishing mill is an extremely simple machine, consisting of a circular horizontal plate of cast iron 14 or 15 inches in diameter, (called a skive,) suspended on a spindle, and capable of being put into rapid motion by means of a larger wheel 5 or 6 feet in diameter, and turned by an assistant.
From the centre to the circumference of the iron plate are lines or shallow grooves formed by rubbing it in that direction with a fine-grained gritstone; these grooves serve to retain the mixture of oil and Diamond powder with which the plate is charged. In order to keep the Diamond perfectly steady while the polishing of each facet is going on, the following contrivance is had recourse to. A copper cup (called a dopp) about three quarters of an inch in depth and in width, and furnished with a stem about four inches long of stout copper wire, is filled with plumbers’ solder, which also projects in a conical form beyond the rim of the cup: in the apex of this cone, the solder being softened by heat, the Diamond is imbedded with one of the facets projecting.
The stem of the cup is now put into very powerful pincers, which screw up with a nut and a wrench or lever, and thus hold it perfectly tight. The handles of the pincers (called tongs) are of wood, are broad and terminated by two feet, about an inch high, so that when laid horizontally they are supported exactly as a pair of candle snuffers is, the studs fixed to the handles of the snuffers representing the legs of the pincers, and the single stud near the point of the snuffers representing the inverted copper cup holding the Diamond, and at the same time having its stem strongly griped by the pincers. In this position the Diamond is placed on the plate, the pincers resting on their legs on the wooden bench or table that supports the plate, and pressing at the same time against an upright iron peg; the broad part of the pincers between the legs and the Diamond, is then loaded with weights, both to steady the machine, and to increase the pressure of the Diamond against the skive.
Matters being thus adjusted, a little oil and Diamond powder is dropped on the plate, it is set in motion at the rate of about 200 revolutions in a minute, and the process of grinding down, and at the same time of polishing is begun. The Diamond is taken up and examined from time to time, and is adjusted so as to give the facet its true form. The heat occasioned by the friction is at all times pretty considerable, and when the pincers are heavily loaclen it occasionally increases to such a degree as to soften the solder and displace the Diamond. This is a serious accident, frequently occasioning a flaw in the Diamond, and always tearing up the surface of the skive so as to damage it very considerably. There is room on the skive for three or four Diamonds at the same time; and to give each its proper share of attention is as much as one person can well manage. The completion of a single facet often occupies some hours.
Diamonds are brilliant cut, rose cut, and table cut. The brilliant is deservedly in the highest estimation, as it is the form which shows to the greatest advantage the peculiar lustre of this gem. The proportions and method of forming the brilliant are described by Jeffries; and with regard to the shape and position of the facets no change has hitherto taken place, although, from the present fashion of preserving as great a spread or surface as possible, the rules for proportioning the dimensions of the brilliant are by no means strictly adhered to.
The brilliant may be considered as formed of two truncated pyramids united together by one common base, the upper pyramid being much more deeply truncated than the lower one. The plane formed by the truncature of the upper pyramid is called the table (a); that formed by the truncature of the lower is called the collet (b); the common base is called the girdle (c); the space between the table and the girdle is the bizel (d) and that between the girdle and the collet is the collet-side (e).
Both the table and the collet are regular octagons; the bizel is formed by eight lozenges and twenty-four triangles; the collet side is occupied by four irregular pentagons alternating with as many irregular lozenges, radiating from the collet as a centre, and usually called the pavilion facets, and bordered by sixteen triangular facets, adjoining the girdle. The brilliant is set with the table side upwards, and the collet side implanted in the cavity made to receive the Diamond.
The regular rose Diamond, is the form given to those stones the spread of which is too great in proportion to their depth to admit of being brilliant cut, without a great loss of substance. It is formed by covering the whole surface of the stone with equilateral triangles, each pair being placed base to base, so as to form a kind of rhomb.
The table Diamond is the least beautiful mode of cutting, and is applied only to those stones or rather fragments, which with a considerable breadth have only a very trifling depth.
Good stones, from one to four or five carats, if skilfully cut either into brilliants or rose Diamonds, lose in the process somewhat less than half their weight; hence the value of a cut stone is twice that of a rough stone of equal weight, independently of the cost of cutting.
The Diamond-cutters of England are confessedly the best in Europe, but their number is unfortunately so small as to, occasion many stones to be sent to Holland ; where, from the greater number and more active competition of the artists, the price of workmanship is considerably lower, but in at least an equal degree inferior to that of London. Brilliant cut Diamonds are so infinitely superior to the others, that of late many rose-cut stones from Holland have been recut into brilliants, notwithstanding the additional expence and the loss of size necessarily attendant on this operation.
Diamonds themselves are always equally in fashion, but the mode of setting them varies according to the caprice of taste or the desire of novelty : hence the jeweller has perpetual opportunities of exercising and displaying the inventive elegance of his taste in the assortment of hues and the arrangement of groups. He will cluster together the smaller stones so as to aggrandize and enhance the effect of the whole; the larger and more perfect ones will generally be set open and displayed to the greatest advantage, while the inferior ones will be assisted by setting them solid on black, or, if need be, with coloured foil.
But whatever be the occasion that calls forth his art, whether the construction of a star, a bandeau, a tiara, a plume, a necklace, or an ear-drop, he will bear in mind that his greatest merit is the concealment of his art: the display of belts and borders of gold can add nothing to the superlative splendour of the Diamond.
Silver fades in the presence of gold, gold itself yields to the more brilliant and costly materials of the jeweller, and of these the most beautiful, the most costly, the very perfection of the gem creation is a colourless brilliant without speck or flaw, large enough to attract notice, yet not so bulky as to be cumbersome in itself, or too disproportionate to the smaller ones with which it is associated.
