. 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. QUARTZ (AGATE): ENGRAVING 521 although they are not esteemed as highly at the present day, yet the number of stones treated in this way is not inconsiderable. The varieties best suited for engi-aving upon, but not devoted exclusively to this purpose, are the onxyes, including onyx pi'oper, chalcedony-onyx, carnelian-onyx, and sard- onyx. These can
. 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. QUARTZ (AGATE): ENGRAVING 521 although they are not esteemed as highly at the present day, yet the number of stones treated in this way is not inconsiderable. The varieties best suited for engi-aving upon, but not devoted exclusively to this purpose, are the onxyes, including onyx pi'oper, chalcedony-onyx, carnelian-onyx, and sard- onyx. These can be so cut as to display a white or light figure against a darker-coloured background. The flatter an onyx is, and the more regular are its layers (Plate XX., Figs. 5a, b), the better suited it is for this purpose; hence Brazilian agates with their plane layers (Plate XIX., Fig. /;) are specially suited for onyx stones, and on that account. Fig. 91. Agate-grinding workshop of Herr August Wintermantel, at Waldkirch (Baden). more valuable. Stones which are to be engraved have the form of plates, and are cut out of even-layered agates, so that the plane of the plate is parallel to the banding of the stone and is of one uniform colour. The uppermost layer is then partly cut away, so that the portion allowed to remain has the outlines of some figure, usually a portrait. It is generally arranged that the figure cut out of the uppermost layer shall be white or light coloured, this being well thrown up by the background afforded by the coloured lower layer. A cameo cut in carnclian-onyx is represented in Plate XX., Fig. 7, and others in Figs. 93 and 94. Sometimes the uppermost black layer is cut through and the figure worked in the underlying white layer, as in the intaglio of Fig. 92; in this case also one gets a white figure on a dark background. A seal-stone of camelian with a letter engraved upon it is represented in Plate XX., Fig. 6. A kind of stone much sought after for cameos is
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