ONYX.	219
	substance of the nail. Some, however, occasionally term marble Onychites, from the mode of testing it (by the finger-nail), or from the purity of its whiteness ; but they are in error." One passage, however, aided by tradition, will afford a clue for tracing the exact meaning of the name amongst the Romans, at least as far as regards the Arabian species. For this is described by Sotacus as differing from the others, in being black with zones of opaque white, whereas the Indian exhibited fiery spots encircled by transparent zones, either one or many around each, but differing from the same marks in the Indian Sardonyx " for in the former these spots are a shade (momentum), in the latter an actual circle." The only conclusion to be drawn from this obser­vation is (with Köhler) that Sotacus called the irregularly stratified stone (our Agate) the Onyx, the regularly stratified the Sardonyx. Now the ring-stone most in favour with the Imperial Romans, next to the fiery Sard, if we may judge from the large number remaining, and the high character of the work they generally present, is a stone of two horizontal layers, the lowest of them black, sometimes opaque, but often tawny by transmitted light, covered by another extremely thin (Pliny's momentum) of milky-white, which, from the reflexion of the dark ground underneath, mostly shows like a turquois-blue. Another Nicolo exhibits the same colours more vividly in consequence of the total opacity of its substance. This arrangement of colours was procured by cutting out one of the " eyes " above men­tioned, together with its encircling zone, and then care­fully reducing it to the form best calculated to preserve the exact distribution of the two shades, by sloping off the sides but leaving the surface perfectly plane. The popular name for such a gem amongst the Romans was AEgyptilla, allusive to its origin, or rather the place of exportation :