Difference between black diamond & black onyx

We all know that black diamond and black onyx are two gemstones coming from different families. Though they are all black in color and look similar in distance, there are actually many differences between them.
Let’s start from the aspect of appearance. The dark black color makes the two gems look similar at some distance. But after some close examination, you will find that black onyx apt to be opaque with a high gloss like a glazed ceramic, while black diamonds are clear even though in black color. Besides, black diamond will reflect light and sparkle and colorless diamonds, but black onyx won’t sparkle like that.
The different appearances result from the different compositions. Black diamond consists of carbon molecule alone, while onyx is a type of chalcedony quartz that consists of silicon and oxygen. That’s why we say the two gemstone come from different families.
The different compositions also result in different hardness. As we know, diamond is the hardest gemstone and rates 10 on the Mohs scale of hardness. Onyx is much softer, rating 7 on the scale. Therefore, a black onyx is easier to be damaged than a black diamond.
At last, two gemstones come at different prices. Compared with black diamond, black onyx is priced much lower and thus more affordable to consumers.
In conclusion, there is nothing alike in black diamond and black onyx except for the same black color. Due to the different composition, the two gems come in different appearance, hardness and price.