Naturalis historia 37.83

37.83

Vitia opalis sunt, si color in florem herbae, quae vocatur heliotropium, exeat aut in crystallum aut grandinem , si sal interveniat aut scabritia aut puncta oculis occursantia. nullos magis fraus indiscreta similitudine vitro adulterat. experimentum in sole tantum: falsis enim contra radios libratis digito ac pollice unus atque idem tralucet colos in se consumptus; veri fulgor subinde variatur et modo ex hoc plus, modo ex illo spargit, fulgorque lucis in digitos funditur.

CHAP. 22.—DEFECTS IN OPALS: THE MODES OF TESTING THEM. Defects in opal are, a colour inclining to that of the flower called heliotropium, or to that of crystal or of hailstones; saltlike grains intervening; roughness on the surface; or sharp points, presenting themselves to the eye. There is no stone that is imitated by fraudulent dealers with more exactness than this, in glass, the only mode of detecting the imposition being by the light of the sun. For when a false opal is held between the finger and thumb, and exposed to the rays of that luminary, it presents but one and the same transparent colour throughout, limited to the body of the stone: whereas the genuine opal offers various refulgent tints in succession, and reflects now one hue and now another, as it sheds its luminous brilliancy upon the fingers.
This stone, in consequence of its extraordinary beauty, has been called "pæderos" by many authors; and some who make a distinct species of it, say that it is the same as the stone that in India is called "sangenon." These last-mentioned stones, it is said, are found in Egypt also, Arabia, and, of very inferior quality, in Pontus. Galatia, too, is said to produce them, as also Thasos and Cyprus. The finest in quality of them have all the beauty of opal, but they are of a softer brilliancy, and are mostly rough on the surface. Their colour is a mixture of sky-blue and purple, and the green hues of the smaragdus are wanting: those, too, are preferred, which have their brilliancy deepened by a vinous hue, rather than those which have their colours diluted, as it were, with water.