Black chalcedony and onyx are two types of silicate minerals that are often confused with one another. Though they share some similarities, they are actually quite distinct. In this article, we'll take a closer look at black chalcedony and onyx, and explore how to tell the difference between the two.
What is Black Chalcedony?
Black chalcedony is a cryptocrystalline form of the mineral quartz. The term "chalcedony" encompasses all cryptocrystalline varieties of quartz, while the "black" descriptor refers specifically to the dark coloration. Black chalcedony gets its color from the presence of various impurities like iron and carbon. It forms in or around volcanic rocks.
Black chalcedony has a waxy luster and smooth texture. It commonly features various banded patterns and colors within the dark matrix. These patterns can range from alternating black and white banding to swirls of gray, brown, and cream. Black chalcedony rates around 6.5-7 on the Mohs hardness scale. It's often cut and polished into beads, cabochons, carvings, tumbled stones, and other ornamental objects.
What is Onyx?
Onyx is also a banded variety of chalcedony, but with very distinct patterning. True onyx displays bands that alternate between black and white. The black bands are pure black while the white bands are an even, milky white color. The banding lines run parallel and are very straight. Onyx forms when silica gel in volcanic lava cools rapidly, resulting in the distinctive banding.
Like other types of chalcedony, onyx has a smooth, waxy texture. It rates around 6.5-7 on the hardness scale. Onyx has long been used for ornamental carvings and jewelry. Black onyx jewelry is especially popular for its bold contrast. Onyx can be found in the United States, Mexico, Brazil, India, and other regions.
How Are They Different?
While black chalcedony and onyx are both banded black and white chalcedony minerals, there are some key differences:
- Onyx has very defined, straight, and parallel black and white banding. Black chalcedony's banding is often wavy, irregular, and less distinct.
- Onyx's black and white bands have sharp contrasts, while the banding transitions on black chalcedony are more gradual.
- Black chalcedony features a variety of secondary colors mixed in, like gray, brown, and cream. Onyx only displays pure black and white.
- Black chalcedony can form in any environment where volcanic rocks occur. Onyx primarily forms in specific lava flows with rapid cooling.
So in summary, true onyx is a strictly banded black and white quartz, while black chalcedony encompasses banded chalcedony with less defined banding and more color variation. Examining the banding patterns is the best way to differentiate the two. Though they share similarities, black chalcedony and onyx are distinct forms of cryptocrystalline quartz.
