Petrosilex, Lapis Cornues, the Hornstein of the Germans; a species of stone claffed by Cronstedt among the siliceous earths. It is of a coars cher texture than the common flint, as well as softer; for which reasons it is not capable of such a fine polish. It is semitransparent at the edges, or when broken into very thin pieces. It is found of different colours, viz. white, whitish-yellow, flesh-coloured, and greenish. According to Mr Kirwan, it runs in veins through rocks, from whence its name is derived; its specific gravity being from 2590 to 2700. In the fire it whitens and decrepitates like flax, but is generally fusible per se. Mineral alkali does not totally dissolve it in the dry way, but borax and microcosmic salt do so without effervescence. Its appearance is duller and less transparent than common flint. The reddish petroflex, used in the count de Lauragais's porcelain manufactory, and there called feldspar, contained 72 per cent. of flax, 22 of argill, and 6 of calcareous earth.

Cronstedt observes that there are not yet any certain characters known by which the cherts and jaspers may be distinguished from one another, though they can easily be so by sight; the cherts appearing of a fine sparkling texture when broken; but the jasper being grained, dull, and opaque, and having the appearance of a dry clay. The chert is also found forming larger or smaller veins, or in nodules like kernels in rocks; whereas the jasper, on the contrary, sometimes constitutes the principal part of the highest and most extended mountains. The chert is likewise found plentifully in the neighbourhood of scaly limestone, as flints are in the strata of chalk.

The connection between these bodies is not yet discovered; but it is impossible to establish any essential difference between them, from the circumstance of flints and agates being generally found in single, loose, and irregular nodules, and hardly in rocks like the chert: for near Constantinople the agate stone runs in a vein across the rock, of the same hardness, and as fine and transparent, as those agates found in round nodules at Deux Ponts.