- Class 2. Earths.
- Order 1. Homogeneous.
- Gen. 4. Silex.
- Spec. 1 Quartz.
- Div. 3. Amorphous.
- Syn. Quartz agathe spheroidal. Haüy, v. 2. 423.
Agate appears to be a very antient name given to this kind of quartzose stone. It is foun on many parts of our shore, as at the Bill of Portland, Lowestoft, and on the Welsh, Scotch, and Irish coasts. It is sometimes found inland, about the lochs in Scotland and Ireland; and, occasionally, in the gravel-pits about London, &c.
This species has been much admired for its resemblance to many oriental stones; and differs from our common pebbles by its toughness, which preserves it from large internal flaws. According to its transparency or colour it is more or less valuable, depending on the state of the owner. Such productions often become pledges of regard, or memorandums of past hours employed in gathering them; and are thus more esteeemed than for their intrinsic value. Exclusive of these social ideas, they are often equal to the best foreign agates, and bear cutting and polishing equally well. We shall speak of the striped and otherwise marked stones hereafter.
The agates found on the sea-coast, being rolled and jumbled together by the force of the waves, are roughened; but being hard, this roughness penetrates but a little way, and the utmost force they experience seems only to make little circular flaws; or, if I may be allowed the expression, more frequently little crescents or semi-circular flaws, from the impulse of the blow coming in a lateral direction.
The right hand specimen is from the Bill of Portland. Its outer surface is generally as here represented, but sometimes whiter. The left hand top specimen came from Lowestoft, and was, perhaps, performed by aggregation, as most agates seem to be (possibly in a trap rock, see Tab. 58), as the cloudy appearance within seems to indicate.
The next figure is of a rougher formation. It was sent by the Rev. H. Davies of North Wales. The smooth one on the right hand, with a little red about it, has been called a carnelian. It was found at Lough Neagh in Ireland: but it must be observed that agates, especially British ones, should no be confounded with oriental carnelians, the fracture of the agate not being so shining, and the stone much harder*. The next specimen on the left is a rather pellucid fragment with the edges partly blunted. The inner figure on the same line was given me by Mrs. Abbot of Bedford, who picked it up in Derbyshire. The smallest of the two lowest ones is apparently a fragment, remarkable for the resemblance to part of a septarium; the inner part resembling the upper right hand figure with a coat of a different colour. The largest figure at the bottom has a resin-like appearance, which these stones occasionally have, and was given me by D. turner, Esq. who brought it from Ireland. Agates that are found in Scotland resemble all these; but what are found there, especially near Perth, are admired for being striped, zoned, forming onyxes, or speckled with various blots, &c. resembling eyes. Mocoas are sort of agate with dendrites or figures like sprigs, trees, &c. which seem to be iron, some say manganese, formed in a peculiar manner with the stones, especially the oriental ones, which are durable; but those called German Mocoas by the lapidaries, seem to have had the branching figures introduced by nature or art into their flaws, and such are apt to disappear, often to the great disappointment of the wearer. We digress a little in speaking of these, which are foreign subjects, as we do not yet know of any stones worthy to be termed Mocoas found in Great Britain.
We consider agate to be nearly of the same nature or a variety of calcedony. It is said to contain Silex 84, Argil 16.
- * This is well know to lapidaries, seal engravers, &c. as it costs them more labour and diamond dust to work them.