Caverdale wrote:shottheradio wrote:What is true onyx? I'm not talking about Onyx marble/mexican onyx. True onyx, I always thought was black and white banded chalcedony? true or false?
My understanding from college minerology classes was that onyx was banded chalcedony, but the bands can be of any colors. In fact, I think black and white only is unusual, but I'm often wrong.
About the same here: I just looked in 3 rock books and 1 gem book: the definition seems to be any relatively straight, wide banded gem-quality chalcedony of alternating colors (as opposed to agate, which often has a circular aspect). Sardonyx always has alternating white and red (white chalcedony and sard, aka carnelian) bands. "Onyx" or "black onyx" usually is cut with the grain of the bands, with an intricate carving in the white part, and the white removed so that the black is the background. Used historically for men's signet rings. Sardonyx is used in the same way for women's cameo carvings.
Bottom line: it's all chalcedony quartz. The calcite onyx superficially looks like the other (banded flowstone) but is too soft and brittle to be used for jewelry.
Posted: Jul 16, 2007 11:39 pm