Like agate, onyx is a type of chalcedony, which is a cryptocrystalline quartz. The key difference between the two stones is that the characteristic bands we admire in agates are curved, while those in onyx are straight, usually alternating between black and white or brown and white.
Onyx is a favorite material for cameo makers, who will frequently carve into, say, a white layer until they’ve removed enough of it to reveal the black layer below. In this case, the result is a white image that appears to float on an inky background. Onyx is also cut into beads for necklaces, teardrops or geometric shapes for earrings, and bold cabochons for rings.
Also like agate, onyx is easy to dye. The recipe calls for boiling the stone in a sugar solution, then letting it steep in sulphuric acid. When the acid hits the sugar, black carbon is created, which seeps into the stone. This technique is used a lot because the demand for black onyx far outstrips the supply. In fact, dyed black onyx was used liberally during the Art Deco era, when dramatic contrasts and hard edges were all the rage.
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