AGATE, sometimes called Scotch Pebble, is an ornamental stone used in jewellery. It is one of the many forms under which silica presents itself, almost in a state of purity : constituting in the agate 08 per cent. of the mineral. It presents a semi-translucent mass with a sort of resinous fracture ; and is some times tinted by a minute quantity of iron. The variations both of translucency and of tint in the same stone are often so great as to give much richness of appearance ; and this combined with the high polish which they are capable of receiving, imparts great value to some specimens of agate.
These stones generally occur in the form of detached rounded nodules, in a variety of the trap rocks called Amygdaloid. The par ticles often arrange themselves in layers parallel to the surface ; and the centre has in some specimens a hollow space containing crystals of other minerals. It is supposed that agates have beeu formed in a kind of lava produced by igneous or volcanic action.
There are many gems which so closely resemble the agate in chemical constitution as to render it convenient to notice them briefly in this place. Carnelian or cornelian is coloured with shades of red and yellow : the deep clear red being the rarest and most va luable. It is brought chiefly from the East Indies, and is much used for engraved seati. Calcedony presents generally a milky white or pale yellow colour, with very often a wavy internal structure. It is sometimes met with in the British isles, in such large masses as to be formed into cups and other vessels. Onyx has the particles arranged in parallel layers : white alternating with blue, gray, or brown. The onyx was much used by the ancients for cameos : the figure or device being cut out of the opaque white, and the dark part forming the ground. A Roman cameo of this
kind, in the Royal Library at Paris, measures as much as 11 inches by 9. Sardonyx is a variety of the onyx, in which the opaque white alternates with a rich deep orange brown of considerable translucency. Mocha Stone is a semi-transparent calcedony, in which varied tints are produced by the presence of iron and other bodies. _Moss Agate closely resembles Mocha stone. Blood-Stone is a green agate, coloured with bright red spots like drops of blood. Chrysoprase and plaima are two varie ties of calcedony having a green tint.
Sir H. T. De la, Beche, in his Anniversary Address to the Geological Society in 1818, drew attention to the artificial colouring of agate. The agate workers of Oberstein are in the habit of imparting colour to that sub stance : an art derived from the Italians. It depends on the difference of porosity in the different layers of the agate. By immersion for some time in honey and water or olive oil, so that the pores of the agate become more or less filled, a subsequent soaking of the stone in sulphuric acid produces a dif ference in the tints of the agate according to the porosity of the layers, the most porous becoming black, while the least porous remain white or uncoloured. By immersion in a solu tion of sulphate of iron, and a subsequent heating of the agate, a cornelian red is in like manner obtained in the most porous layers, while the least porous remain unchanged in colour. It is supposed that some of the agates Which have come down to us from antiquity have been artificially coloured.
In the Exhibition of works of Medieval Art, at the rooms of the Society of Arts in 1850, many exquisite specimens of agate were collected.