Black onyx and black sapphire have very different properties, even though they are similar in color. Careful observation can sometimes determine if a black stone is an onyx or a sapphire, but a gemologist would be needed to determine it by hardness and chemical composition. Knowing the differences can help avoid confusion in the purchasing or selling of jewelry.
Onyx is rated at a hardness level of 7. Sapphire has a hardness rating of 9, making it significantly harder and stronger than onyx. This means the stone is less likely to crack, chip, or break during wear.
Onyx is a banded stone, which means it is not a solid color but has several bands of usually white and black. Sapphire has a more uniform color, although different hues and depths may be seen in it. Onyx also tends to be a solid stone that won't allow any light to pass through it. Light can easily pass though the translucent sapphire.
Onyx can be found in Scotland, the New England states, and in the North-Central America. Sapphire is far more common and can be found in many more places, including France, Germany, Russia, Scotland, Pakistan, Burma, Kenya, Colorado, Connecticut, and North Carolina.
Sapphire is made of an Aluminum Oxide. The chemical formula for sapphire is Al2O3. This differs vastly from the chemical composition of Onyx which is SiO2, a form of silicone dioxide known as an agate.
The value of both stones varies based on the cut, the setting, the color and the quality. In 2010 the selling price of the average caret of sapphire was $120 while the average price of the onyx of the same size and quality costs $1.25 according to AlphaImports.com. Sapphire is the more popular jewelry stone because of its light catching properties while onyx is commonly used for relief jewelry.