Black diamond and black onyx are two different stones, coming from different gem families. The two stones, upon close examination, look very different in appearance. The stones also have different properties and compositions, making for a variety of ways to differentiate between the two.
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.
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.
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.
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.
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.