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180 A Book of Precious Stones
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donyx and carnelionyx derive their names from the colours of the intervening layers. " Mexican onyx," it should be noted, is calcite, not quartz, and is very much softer than the real onyx. Mexican onyx has a similar banded structure to real onyx, and is well adapted to architectural or interior decoration, for which it is extensively used, but it is outside the realm of precious stones.
Because of their porous nature, varieties of agate can be easily artificially coloured, and this art has been developed to perfection in Germany, where some of the processes, as " trade secrets," are important phases of the general agate-preparing industry at Oberstein and Idar. The art of colouring agate, which naturally is mostly of a dingy grey colour, was derived from old Rome. Brazilian agate, the material extensively worked now in Germany, is softer than the German varieties that formerly constituted the principal supply, and is particularly susceptible to successful colouring by the scientific German processes.
The onyxes best suited for cameo engraving, besides onyx proper, are chalcedony-onyx, carnelian-onyx, and sardonyx. These are cut
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