. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. Fig. 94. Antique cameo. {See page ; Fig. 93. Antique cameo. separation of carbon, which imparts its black colour to the stone. The " softer " stones,, even after a few hours immersion in the acid, acquire a deep black colour, others require an immersion lasting for a day or several days, and some pieces only assume the black colou
. Precious stones, a popular account of their characters, occurrence and applications, with an introduction to their determination, for mineralogists, lapidaries, jewellers, etc. With an appendix on pearls and coral. Precious stones; Pearls; Corals. Fig. 94. Antique cameo. {See page ; Fig. 93. Antique cameo. separation of carbon, which imparts its black colour to the stone. The " softer " stones,, even after a few hours immersion in the acid, acquire a deep black colour, others require an immersion lasting for a day or several days, and some pieces only assume the black colour after being immersed for a considerable time. When the stone is as black as it is possible for it to be, it is taken out of the liquid and quickly dried in an oven. It is then cut and polished, and finally rubbed with oil or allowed to soak in this liquid for a day, the oil not absorbed being wiped off with bran. This final treatment hides small cracks and improves the lustre of the stone. The fine black agates now bought and sold in the markets have been artificially coloured in this way, as have also the onyxes (Plate XX., Figs. 5a, h), in which the black layers, now alternating with the white, were originally greyish or bluish in colour. Not only do highly porous layers become a deep velvety black, but less porous bands acquire a more or less dark shade of brown. The shade of colour impai'ted artificially to a stone depends, to some extent, on its original colour; a red layer, for example, after being artificially coloured will have a reddish tinge, and so on. It is a significant fact that, according to the way in which the stone will acquire the black colour, a hundredweight of agate may be worth £5 or ^250. When offering such stones for sale, it is therefore customary to submit small fragments as samples for the purpose of testing whether they will acquire a good colour. The tinting of agates with colours other than black is comparatively unimportant, but a few details
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