Fictitious gems are prepared in a variety of ingenious methods, sometimes stones of inferior value are modified in their colours by heat, and substituted for more valuable gems, as in the case of the zircon, which is sometimes rendered colourless by heat, and substituted for the diamond. The colours of carnelian are principally given by heat. In Phillip's Mineralogy, it is stated, that carnelians, when found, are of a blackish olive passing into a grey. "These are first exposed to the sun for some weeks, and then placed in earthen pots, and subjected to heat, which gives them the colours which constitute their value in jewellery."

Carnelian, when imported into England, is generally of a red colour, and when the colour is too light, it is sometimes deepened by putting it in an iron pot, and gradually bringing it to a red heat. Yellow carnelian is by the same means rendered red; but the effect of heat upon white carnelian is to render it more opaque, and advantage is sometimes taken of this circumstance to give white carnelian the appearance of the white onyx. The various colours of agates are, in some cases, more fully developed in the same manner; at other times, the agates are soaked in oil for two or three hours; the oil penetrates the agate, and is afterwards carbonised within the stone by exposing the latter to the fumes of heated sulphuric acid. Nitrate of silver is also sometimes employed for staining agate, and other stones.

Pastes are, however, the most frequent substitutes for precious stones. The foundation of all pastes is a superior colourless glass called strass, made from very pure materials, and afterwards coloured by the addition of metallic oxides, in much the same manner that ordinary coloured glass is made, except that the process is more carefully performed throughout. The French are considered to excel in the preparation of pastes, and a variety of recipes for the manufacture of pastes, derived from various French authors, are given in Dr. lire's "Dictionary of Arts," p. 943, and also in Gill's "Technical Repertory," vol. ii., p. 308.

Metallic foils made of thin sheet copper silvered and burnished, and afterwards coated with transparent colours, mixed with isinglass size, are often employed by jewellers to improve the brilliancy of pastes and inferior stones. The foil is enclosed in the setting, and entirely covers the back of the stone, to which it imparts much of its own brilliancy. When it is desired to modify the colour of the stone, a foil of a lighter or darker tint is used, according to circumstances. Crystals and pastes, set as imitation diamonds, generally have a piece of silvered foil at the back.

Painting is sometimes resorted to for counterfeiting topazes, and other gems; in this case, a colourless stone, such as crystal is employed, and the back of the stone to be enclosed in the setting, is painted with the colour removed from a piece of foil, and another piece of the same foil is placed behind the stone in the setting, to improve the brilliancy. The reflection of the colour from the back of the stone is so uniformly diffused throughout its substance, that, even upon close observation, the unpractised eye fails to detect the absence of colour in the body of the stone. In removing the colour from the foil, the latter is gently warmed over a candle, and, while warm, the colour is worked up with a moistened brush, and immediately applied to the stone, care being taken to cover every portion of the back, particularly the angles formed by the meeting of the facets, as, should the smallest speck remain uncoloured, it would reflect a ray of white light that would altogether mar the effect. The painting of these fictitious gems is sometimes so successfully executed, that only those persons thoroughly conversant with precious stones, are enabled to distinguish between the real gem and the counterfeit, so long as the stone remains in the setting.

Doublets are a more elegant and substantial application of the method of counterfeiting gems by coloured backs, and transparent fronts. In doublets, the front and back are made in two pieces, cemented together on the line of the girdle. The front is made of a colourless stone, and the back of a coloured paste; the two surfaces to be placed in contact, are first ground quite flat and smooth, to fit each other accurately; they are then cemented together with a very thin layer of clear mastic, and the doublet thus prepared is cut as a single stone.

In real gems, advantage is sometimes taken of the power of a coloured back to give colour to a colourless front. It occasionally happens that a gem may be partly colourless and partly coloured. In this case, instead of dividing the stone, the coloured portion is placed at the back, and, if possible, the stone is so cut that the table shall be parallel to the imaginary line dividing the two portions; and if the stone has much natural brilliancy, it is not imperative that the coloured portion should extend to the girdle, as a comparatively small piece, properly placed, will serve to colour the entire stone.

A striking illustration of this was recently observed by the writer in the case of a sapphire, the bulk of which was perfectly colourless, and a small part only of a deep blue colour. This gem was about one-sixth of an inch in length, and nearly of the same measure from the table to the culasse, the great proportional depth having been adopted, in order that the colour might be reflected throughout the body of the stone, from the small blue portion, which was scarcely larger than the head of an ordinary pin, and yet in consequence of its being situated exactly upon the culasse, and extending a small distance up the back facets, the blue colour appeared to be uniformly diffused throughout the stone when viewed from the front, although at the time of inspection the stone was not set. When viewed from the back, the body of the stone appeared quite colourless, with a small speck of a dark blue colour on the culasse.

Some management is, however, required to obtain this result, as, if the stone have too much width or spread, the edges will appear colourless, and in extreme cases the blue will only show as a dark central speck. Owing to the various degrees of brilliancy in different stones, and the variation in the size of the coloured portion, no invariable rule can be adopted for the spread of the stone relatively to the depth; the proportions are therefore obtained by trial, the spread being gradually reduced without interfering with the thickness, until the desired result is obtained.