A TABLE OF GEMS.

Continued from p. 214.

Clafs twelfth.

OPAL and CAT's EYE.

The PHAROS of the Greeks

HARDNESS IO; SPECIFIC GRAVITY from 1,7 to 2,2.

Varieties.

The SANGENON of the Indians, and NONNIUS of the ancients, is of an olive colour, reflecting ruby coloured rays; the IRIS OPAL of a glafsy white, reflecting greenish, yellow, purple, and bluish rays; MILKY OPAL, pale flame coloured rays; CAT'S EYE, redish colour, almost transparent; AVANTURINE CAT'S EYE, contains little plates of a metallic splendor; DARK GREEN, purple and green rays; OLIVE, greenish yellow rays; WHITE, blue rays; BLACK, white rays; BLUE, white rays; VIOLET, white rays; YELLOW, white rays; GIRASOL, weak bluish rays with an orange yellow; FISH'S EYE differs from the cat's eye in its colour, which is bluish.

Analysis.

ORIENTAL OPAL, Sp Gr 1,9; Arg 4; Sil 47; Cal 2; Ir 9; Mag 38. CAT'S EYE from Eilbenstock, Sp Gr 1,7 to 2,2; Arg 15; 8.1 84 *• OCCULUS MUNDI, Sp Gr 2,c.

Form.

The form of the opal is that of a pebble, like the agate, with which the authors in general have classed it, from a supposed resemblance, which I find no sort of proof of. On the contrary, Bergman's analysis points it out to be of a very different nature

from the genus of flints, of which the agate is a species; magnesia constituting a large part of its composition, and not entering at all into that of the agate, if we are to judge from the analysis of the parent species or flint, there being none yet published of agate. The specific gravity of the opal is likewise extremely different from that of the agate. These reasons appeared to me sufficient for taking one of the most beautiful and valuable gems from the second order, where it had been placed, and putting it into the first, which appears its proper station, till better ar guments can be urged for disgracing it. The olive coloured opal, is the sangenon of India, and Nonnius of the ancients and modern Europeans, from the Roman senator Nonnius, possessor of the famous opal of Rome, worth 20,000 sesterces, who preferred banishment to parting with it to Anthony. An opal answe◄ ring exactly to Pliny's description of the Nonnius was discovered about thirty or thirty-five years ago in the ruins of Alexandria, and purchased for a trifle by the French consul Lironcourt, from his draguman Roboly. The duke de Nivernois, when ambassador in London in 1763, was in pofsefsion of the very stone. The next in esteem and value is the Iris opal, of a glafsy white colour, but when looked through appears of a flame colour, as the Nonnius does of a ruby. Wallerius thinks the Iris is meant by Pliny, instead of the former for the Nonnius. It reflects green, yellow, purple, and bluish rays, whilst the first reflects ruby coloured. The milky coloured opal throws out pale red rays. The bluish coloured opal is the most common and least valued of any. The girasol exhibits in its internal part a luminous point,

and reflects the rays of light in whatever position it may be turned, when it is cut into a globe or hemisphere.

Structure, Properties, &c.

Gems and chrystals are of a laminar or foliated texture, or composed of thin transparent plates, laid regularly one upon another, and adhering strongly ; but when it so happens that these little plates or lamina, instead of lying in the regular manner described above, which constitutes the common laminar texture, are of a broken texture, like the opal, in clusters, differently inclined to one another, they then reflect various colours according to the angle you view them in. The opal is semitransparent, although it appears opaque by reflection. It is affected by fire like flints, and melts with borax with difficulty. The oriental are said to be very hard, but the occidental is only ten in this table.

THE CAT's EYE.

This is a variety of feldt-spath, found in a pebble form, like the opal; and I have given it a place in the first order of gems, merely because it is esteemed and worn as such,-some of the specimens which I have seen being of an extraordinary price; such as one belonging to the late prince Potemkin, said to have been taken from the orbit of a Gentoo idol, of which it made one of the eyes, by the famous Nadir Shah, and disappeared, with many other gems, from his tent, when that conqueror was afsafsinated. The other eye has never been heard of as yet: probably it may have found its way back to India. The ba ron Turbe, ambassador at the court of St Petersburg

Feb. 13. from his Sardinian majesty, has a most uncommon `variety of great value, of a dark green or bottle colour, reflecting purple rays, concentrated in a round spot, like the pupil of the eye, and surrounded with a light green circle. The author of this table had an olive coloured cat's eye in his possession, less than any of the two mentioned above, and much inferior in every respect, for which a jeweller paid him 600 rubles to set in a ring. The good cat's eye is in general of an olive colour, and throws out bright greenish yellow rays, like the real eye of that animal; of this kind was that of prince Potemkin, and the one disposed of by the author. The FISH'S EYE only differs from the cat's eye in being of a bluish colour.

Structure, Properties, &c.

The texture of the cat's eye is laminar, with the edges of the plates visible to the eye, a particular construction, which produces what the French call des pierres chatoyantes, or stones that change colour according to the position you view them in, like certain well known silks. Several varieties of feldtspath pofsefs this property in an eminent degree, and produce a number of curious and beautiful stones described in the next order. It is rather surprising that some celebrated mineralogists clafs the cat's eye with the under the name of the Pseudo

opal.

agate,

Where found.

The sangenon or Nonnius opal, is found in the

East Indies; the Iris, in Ceylon;

the milky o

pal, at Eilbenstock and Freyberg; the bluish or

most common and least esteemed, in Hungary, Silesia, Saxony, &c.; the olive and bottle coloured cat's eye, in Ceylon; the inferior in different countries of Europe. Mr Børn mentions what he calls an avanturine cat's eye, of a flesh colour and transparent, possessing the curious structure of the avanturine, viz. composed of little plates like scales, with a metallic splendour, which reflect the rays of light like the opal. This stone I suspect to be that which has led authors to clafs the avanturipe with the opal, although it is in fact a fine opaque quartz, as will be mentioned in the second order. Russia produces the opal at the rivulet, Katscha, near the city of Krasnajark, in the Altai mountains in Siberia. The cat's eye is found in mount Caucasus. Although the occulus mundi is not intended by the author of this table to be introduced into the first order of gems, though highly curious, and of course is not named, still there is no place so proper to mention it as the article of the opal, with which it has so intimate a connection, from its being generally found in beds over it; and from being regarded by some naturalists, as the same stone in a state of decomposition by the action of the air. It has the curious property of becoming transparent when kept in water a few hours, although naturally opaque, and of a whitish brown colour. This phenomenon proceeds from its absorbing water, in the proportion of three grains to thirty-five of stone, when fully saturated. When it is saturated with oil of vitriol, instead of water, it preserves its transparency for a length of time, by the