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Appearance
Onyx tends to be opaque, sometimes with a high gloss like a glazed ceramic. Black diamonds will be clear, even though they are dark in color. Black diamonds will also reflect light while onyx has no prismatic reflections.
Composition
A diamond is composed solely of carbon. Onyx is a form of chalcedony quartz, meaning it is composed of silicon and oxygen. These two gemstones share no common elements or aspects of composition.
Classification
Since a diamond is made only of carbon, it is classified as a native element. However, onyx is classified as a tectosilicate like other varieties of quartz.
Hardness
A diamond is the hardest gemstone, rating a 10 on the Mohs scale of hardness. Onyx rates at a seven, making it softer and easier to damage than a black diamond.
Prices
Black onyx is priced considerably lower than black diamonds. Onyx is a much more common gemstone and also less prized by cultures and consumers. However, these two stones are nothing alike, other than the fact that they share a coloring. The cost of black diamonds, like all diamonds, depends on the quality of the individual stone and location of purchase.