Agate, variety, types of agate

Agate, variety, types of agate

It is the agate, and the varieties of which it is the type, that have in all ages furnished to the engraver the stones best suited to his art.

One of the most remarkable engravings upon agate, and one of the finest specimens of this stone, is represented by Fig. 74. It is the bust of Alexander the Great. The head is carved in relief, and its colour is quite different from that of the groundwork of the stone. It is set in a superb frame of enamelled gold.

chalcedony displaying a bacchic bull with an ivy wreath around his body and a thyrsus under his feet. It is one of the most celebrated of antique engraved gems, and bears the signature of the famous graver Hyllus.

As a specimen of modern engraving, we represent by Fig. 76 that celebrated stone known as the “seal of Michael Angelo.” It is a small transparent cornelian engraved en creux or entaille. In the small space of an oval, hardly more than half an inch in length, there are fourteen figures, besides the scenery of a river with water-monsters and a fisherman. It is a bacchanalian or vintage scene, and it recalls a part of Michael Angelo’s fresco of “Judith committing the head of Holophernes to her attendant.” Critics are at variance concerning this cornelian: it has been ascribed to the famous engraver Pyrgototes, with the supposition that Michael Angelo used its design as a passage of his great fresco; and, on the other hand, it is called a modern chef-d’oeuvre, whose engraver has been inspired by Michael Angelo.

This stone was in the cabinet at Versailles, and was one day swallowed by an enthusiast in gems; but fortunately Hardion, who was exhibiting the treasures, observed the act, and before the honest man departed persuaded him to take an emetic for the benefit of his stomach. The gem was in this manner immediately recovered.

The agate, unlike other precious stones, very rarely occurs in veins; it is almost always in the state of concretions; sometimes in the form of geodes or balls. Occasionally there is found in the side of one of these balls a sort of funnel through which the silicious matter was introduced. Sometimes the gelatinous silica has been abundant enough to give rise to homogeneous deposits of a certain depth; the stone in that case is of uniform colour; but often the deposits are in very thin layers, and of different shades of colour; often, too, they are moulded by the cavities of the body which forms their support, and take from its irregularities all sorts of dispositions with very variable shadings.

In cutting a section across a stone of this category, extremely different effects are obtained by following different directions. The varied zones and colours of the stone produce, too, infinite varieties; and descriptions names have been bestowed upon agates, according to these changes, as rainbow, cloud, moss, star, ruin, landscape, fortification agate, &c. The differences between all these varieties are extremely slight in a physical or chemical point of view.

Agates are divided naturally into two varieties:–

Agates of a single tint.

Agates of several tints.

FIRST VARIETY.

Chalcedony.–The chalcedony is quite a common stone, of a dull or milky-white; and sometimes of a bluish tint, when it is called saphirine.

The ancients obtained chalcedony from Egypt and Syria, and it was an object of considerable commerce at Carthage. It probably derived its modern designation from Karchedon, the Greek name for Carthage. It is found in England, Ireland, Germany, Italy, &c.

Chrysoprase.–Chalcedony coloured by the oxide of nickel, varying in colour from deep verdigris to the palest green. It takes a very beautiful polish, and fifty years ago was fashionable in jewelry, though now quite forgotten.

Cacholong.–A variety of chalcedony of a whitish tint, cloudy almost to opacity. It is found in Bokhara, Ireland, Greenland, and the Faroe Islands.

Cornelian.–A species of chalcedony, but of a finer grain. The ancients confounded it with the sardoine, and it was not until the thirteenth century, in the writings of Albertus Magnus, that the distinction became established.

The cornelian has sometimes the colour of polished horn; there is one variety that resembles the hyacinth, and another, of vermilion red, some-what analogous to the ruby. Its colouring is due to the oxide of iron, and in certain varieties to organic matter, clearly discernible by analysis.

Heliotrope.–A translucid agate of a lively leek-green colour, spotted with red. The ancients used it, as Pliny tells us, for looking at eclipses of the sun, as we use smoked or stained glass; and it was said also to change the colours of the sun’s rays into blood-red, when it was plunged into a vase of water. Hence the name heliotrope, from the Greek helios, the sun; and trepo, to turn.

SECOND VARIETY.

Onyx.–The onyx is the most celebrated variety of all the variously tinted agates.

Originally the name onyx was given to agates which had the appearance of a nail (Greek, onyx) where it joins the flesh; but it is now used for stones which exhibit marked contrasts of colour in bands, as black and white, or black and whitish-gray.

When an onyx unites in a desirable degree these conditions, it constitutes a stone of value, on account of the resources offered by it to the engraver, through the contrast of colours.

Sard.–This word, very anciently used, is said by Braunius to be derived from the Hebrew sered, “a red colour.” However this may be, it is to agates of this colour that the name is applied.

 

Sardonyx.–A stone formed, as its name implies, of the sard and the onyx, using the latter term in its primitive signification. The sardonyx is a stone displaying alternate layers successively whitish and carnation red.

Sardoine.–Considered by many mineralogists as identical with sardonyx. Engravers, however, recognize between these two stones a marked difference: for them, the sardoine is an agate whose deep colour partakes both of yellow and red without either colour predominating. In colour, therefore, the sardoine differs completely from the sardonyx.

Sard-agate.–A semi-transparent stone formed of an inferior layer of orange-red or pale yellowish-red, and a superior layer of whitish tint, disposed one upon another with perfect regularity.