In the art world this expression has a far wider bearing than in strict scientific language. It answers very nearly to the term "stones" of the old works on natural history, works which merely distinguished between fine, hard and soft stones. The first, to which since the thirteenth century the name of gems has been restricted, are technically all such as are cut, and more especially those employed in jewellery, embroidery, etc. The hard stones, occurring in more or less considerable masses, are also subject to the process of cutting, but are less pure than the fine stones, though much more sought than the soft stones, the cutting and polishing of which presents little difficulty.

It may be useful here to give the essential characteristics of the gems most commonly diffused in the art world, whereby amateurs may be the more readily put upon their guard against the errors involved in certain current phraseology in opposition to scientific definitions. It is more specially applicable to the goldsmith and jeweller's art, what we shall have to say on the nature and cutting of fine stones, yet these observations have a right to a place here in connection with the artistic engraving, particularly of cameos or intagli, their rarest forms.

A word at the outset on these terms. Now the word stone cameo is used more particularly when the figures in relief are of a different colour from the ground. In the glyptic art, the name simply expresses this relief, whether the stone be colourless as rock crystal, or uniform in tone as the beryl or amethyst, or in more or less numerous layers of different colours as we see in all the varieties of agate from the bluish "Nicolo" onyx with its deep brown ground to the sardonyx where the artist may employ as many as six distinct layers, superposed, as may be seen in the famous agate of the apotheosis of Augustus.

Intaglio, on the contrary, implies an engraving hollowed out. Here in the case of hyaline gems, the work is not always presented to view by the face on which it has been executed, but by the reverse, which is smooth, a crystal intaglio assumes the appearance of a cameo. The art was thus practised in the East as early as the sixth century, and in the West at the Renaissance.

In classic times, when the passion for gems was pushed to excess, certain stones, such as the emerald, were considered too choice to be submitted to the engraver. Others again, as for instance the diamond, were never engraved, because the ancients, who knew how to employ the diamond in cutting other stones, never hit upon the device of using it for cutting the diamond itself.

Of all peoples the Egyptians have shown the greatest skill in the use of hard stones, such as porphyry, basalt, granite, etc.

The question has been raised whether the processes of cutting gems had perished in the West during the middle ages. After serious study, M. Jules Labarte came to the conclusion that such was the case, resting upon arguments apparently unanswerable. It is certain that all the precious stones used at this epoch are in the form of cabochon, and that those however slightly engraved are in fact of ancient origin. Still certain doubts are suggested by a piece of rock crystal, the elegant lagena mounted in gold, known as the vase of Queen Eleanora. According to the inventories, the crystal forming its vase would be of ancient workmanship. Yet in this cellular, irregular and rudimentary piece, it would be difficult to recognise the workmanship either of Latin or of oriental artists, skilled as both were in the cutting of stones. In our opinion this piece, which is clumsily hollowed out and of enormous thickness, is the work of an art seeking to recover itself.

Let us now pass on to the character of gems.

Sapphire

Composed essentially of alumina, this stone scratches all others except the diamond. The sapphire or hyaline corundum, the most commonly diffused, is blue; yet there are some colourless, which are occasionally passed off as diamonds. Other colours, occurring more rarely, are the crimson red designated as the oriental ruby, yellow or oriental topaz, green or oriental emerald, which tint does not approach the emerald of Peru, lastly the violet or oriental amethyst, generally of a clear tone.

Topaz

Topaz , fluorated silicious alumina, scratches quartz and is scratched by the ruby. Some varieties are colourless, but the Brazilian topaz is of a deep orange yellow. When subjected to the heat of a sand bath or burnt in a covering of amadou, it changes colour and assumes a beautiful rose tint. It is then called Brazilian ruby, and becomes more valuable than before combustion. The straw-coloured topaz of Saxony is on the other hand rendered colourless by the action of fire.

Zircon

Zircon , formerly called Hyacinth, a combination of silica and zirconium, scratches quartz with difficulty. Owing to its feeble lustre it is held in little repute and presents two varieties - a pale yellow, known as the jargoon of Ceylon, and a brownish orange, the hyacinth; the last employed somewhat frequently in ancient times.

Chalcedony

Chalcedony , an agate quartz of a cloudy transparence, and a somewhat milky colour with yellow and bluish tints. When polished it often exhibits ruddy spots or veins, and little rounded cloudings due to the nodules in which it occurs. When so found, it is known as oriental agate.

Sardonyx

Sardonyx , a quartz agate of a more or less decided orange hue, in pieces of a certain thickness, passing to a chestnut brown. It is found in China and Siberia, in the form of pebbles, frequently conchoidal on the surface.

Of this material have been made most of the finest antique cameos, notably that of the Sainte Chapel le, representing the Apotheosis of Augustus. It is in five layers, and it not above eleven inches high. Sardonyx is also the material of the Cup of Ptolemy, another precious relic, preserved like the previous, in the Paris Cabinet of Medals. With its handles, it measures nearly twenty inches in diameter.