in natural history, a species of the chroa-flaces genus of gems.—This species of precious stone is generally esteemed the most beautiful of all the flinty tribe, which appears to be owing to its changeable appearance when viewed by reflection.—The form of the opal is that of a pebble, like the agate, with which authors in general have clasped it, from a supposed resemblance, of which there appears no sort of proof. 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. Wallerius tells us that its specific gravity is upwards of 1900. It loses its colour and transparency in the fire, and in other respects is affected by it in the very same manner as quartz or flint would be. It may be melted with borax, but not without great difficulty. The species are:

1. The opal of Nonnius. This appears olive-coloured by reflection, and then opaque; but when held between the eye and the light, it is found to be transparent, and appears of a beautiful ruby colour. Bocede Boot, author of the *Complete Jeweller*, considers it as the most precious sort of opal, and indeed the most wonderful of this kind of nature's works: he gives a lofty encomium upon it, chiefly from Pliny, who called this opal *pædæra*. This species of opal is the *fægemon* 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 answering exactly to Pliny's description of the nonnius was discovered about 30 or 35 years ago in the ruins of Alexandria, and purchased for a trifle by the French consul Lironcourt, from his dragoman Roboly. The duke de Nivernois, when ambassador in London in 1763, was in possession of the very stone. The next in esteem and value is the Iris opal, of a glassy white colour, but when looked through it appears of a flame-colour, as the nonnius does of a ruby.

Wallerius indeed is of opinion that the opal found in Alexandria was not that of Nonnius mentioned by Pliny; and adds, that it was by many supposed to be only a counterfeit piece of glass or patte. There is another of the same species in Sweden, which by reflection appears of a brownish colour, but by refraction is red with violet veins.

2. The white opal, having its ground of a white glass-like complexion, from whence green, yellow, bluish, and purple rays are thrown out; but when held against the light it appears of a reddish or rather flame-colour. Wallerius, in his Mineralogy, says, that this white opal answers the description of it given by Pliny much better than the olive coloured one above described. There are two varieties of it: 1. The oriental opal, showing many colours.—Engenstrom informs us, that he had obtained a small piece of pseudo agate from the East Indies, of a yellowish brown and pale blue, or rather milk colour, with a shining brightness, exactly like that of the milky opals already mentioned; also some other specimens near Turin in Piedmont, where they are called *baillard-agates*, a name which, in his opinion, is extremely proper for them, as they agree with the agates in almost every respect except hardness: this, however, has been controverted—Sometimes the opal is surrounded with a white crust, like common flints in the strata of chalk; which crust has likewise the same properties as the flint when the last mentioned substance has been previously freed from the adherent chalk; viz. 1. It does not dissolve in nitrous acid. 2. It is not fusible per se. 3. It melts pretty easily with borax, but without any effervescence, contrary to what is observed in calcareous substances; so that borax will dissolve about three quarters of its own bulk of this substance, though with difficulty, especially towards the end of the operation; but the glass becomes quite clear and colourless, instead of becoming white and opaque, as is the case with calcareous substances. This oriental stone is found in the island of Ceylon, where it is called the *elementary stone*. The Indians put as high a value on it as on the diamond. There is another kind of oriental opal much valued, generally called the *flaming opal*, because it changes its colours, as if sparks of fire escaped from it in parallel lines.

3. The bluish and semitransparent opal is less valued by those who are conversant in gems than the others, on account of its being supposed more easily imitated by art. M. Magellan, however, informs us, that not only this, but several other kinds of opals are easily imitable by art; several compositions of glass being met with which show very different colours by reflection and by refraction. A curious ancient one of this kind is to be seen in the royal abbey of St Denis near Paris, which is green on the outside, but shows a fine ruby colour when held between the eye and the light. Our author has also seen some glass pastes made in London by Edward Delaval, Esq., and others by Mr More secretary to the Society of Arts, which appeared of a yellow-brown or other colour by reflection; but when held against the light transmitted a fine blue, purple, or red colour, like the sapphires, rubies, garnets, and other precious stones.—Wallerius gives directions for making these pastes; and M. Magellan informs us, that he by chance discovered that the red glass of Kunckel, when over-melted, or burnt in a common fire, produces a similar effect, transmitting one colour by refraction and another by reflection. The fine imitations of the true white opals, which Pliny says were made by the Indians, have, in our author's opinion, hitherto baffled the art of the moderns. The sanguine or nonius opal is found in the East Indies; the Iris, in Ceylon; the milky opal, at Eilbenstock and Fryberg; 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. Born mentions what he calls an avatrine cat's eye, of a flesh colour and transparent, possessing the curious structure of the avatrine, viz. composed of little plates like scales, with a metallic splendour, which reflect the rays of light like the opal. This stone we suspect to be that which has led authors to class the avatrine with the opal, although it is in fact a fine opaque quartz. Russia produces the opal at the rivulet Katicha, near the city of Krasnajark, in the Altai mountains in Siberia. The cat's eye is found in Mount Caucasus, and is often confounded with the opal, though improperly. See Astoria. The oculus mundi (see Hydropianes) has a very intimate connection with opal, being generally found in beds over it, and being regarded by some naturalists as the same stone in a state of decomposition by the action of the air. Russia possesses this stone in the Altai mountains, where the opals are found.

No method of estimating the opal is given by authors that we know of. But those of uncommon beauty and size are sold for very large sums.

The late Leopold II., emperor of Germany, was in possession of an oriental stone, sometimes described as a cat's-eye and sometimes as an opal, of one inch diameter, and which was valued at a great price. Prince Potemkin, the Russian general, purchased for 1000 ducats a stone of the same kind, said to have been taken by the famous Nadir Shah from the head of a Gentoo idol, of which it made one of its eyes. By what circuitous road it found its way to Potemkin, we have not been informed; but with many other gems it disappeared from the tent of the Persian conqueror when he was assassinated.

Opals are commonly found in detached pieces, in an envelope of a different kind of stone, from the size of a pin-head to that of a walnut. Beautiful opals of this last size are extremely rare; so that it is difficult to find an opal sufficiently perfect and large to be completely possessed of all its beauties: this renders it so precious, and makes it almost impossible to determine its value. They have agreed, however, to value a beautiful oriental opal at double the price of a sapphire of the same size.

It is very remarkable, that all the beautiful colours of the opal may entirely change or disappear when the stone is divided into pieces. This phenomenon, which has been demonstrated more than once by experience, leads us to think that all the sparkling play of the opal is owing to the refraction of the rays of the sun from the surface of the stone, which is naturally formed to produce this refraction.