Onyx and agate are often confused because they are close relatives within the world of mineralogy. Both onyx and agate are varieties of the same fundamental mineral material, meaning they are chemically and structurally similar. This shared parentage often causes confusion, but the scientific distinction between them is clear and based entirely on a single visual characteristic. Understanding the classification of these stones requires looking at the microscopic structure of their shared composition.
The Shared Parent Material: Chalcedony
Both agate and onyx belong to the family of minerals known as chalcedony, which itself is a microcrystalline form of quartz. Chalcedony is composed primarily of silicon dioxide (SiO2), the same chemical formula as common quartz. The term microcrystalline means the individual quartz crystals are so small they can only be seen with high magnification, giving the material a smooth, waxy luster rather than the glassy appearance of large quartz crystals.
This structure, which often includes a small percentage of the mineral moganite, is responsible for the toughness and durability shared by both gemstones. Chalcedony forms when silica-rich water fills voids within existing rock, such as the gas bubbles in cooling volcanic lava. As the water evaporates or cools, the dissolved silica precipitates out and deposits layers onto the cavity walls. This layering process is what gives rise to the various banded chalcedony varieties, including agate and onyx.
Agate: Defined by Concentric Bands
Agate is a banded variety of chalcedony that is typically translucent. The defining characteristic of agate is the pattern of its colored bands, which are most often curved, concentric, or wavy, following the contours of the cavity in which they form. This type of formation is known as wall banding, where the silica precipitates sequentially from the outside inward toward the center of the void.
The rich diversity of colors found in agate is a result of various trace mineral impurities present in the silica-rich water during the deposition process. For instance, iron oxides can create red, orange, and brown hues, while manganese can introduce black or pink coloring. The alternating bands of color and transparency are what make agate popular in jewelry and decorative objects.
Onyx: Defined by Straight Parallel Layers
Onyx is a specific type of agate distinguished by the geometry of its banding. The difference is that onyx features bands that are perfectly straight and parallel, rather than the curved or concentric bands typical of other agates. This straight-line layering, also known as level banding, occurs when the silica precipitates horizontally within a cavity, often following gravity.
The name “onyx” comes from the Greek word for “claw” or “fingernail,” referencing the stone’s distinct, parallel lines. True onyx is characterized by alternating bands of light and dark colors, most famously black and white. A well-known variant is sardonyx, which features parallel bands of white alternating with shades of red or brown. Much of the solid black onyx seen commercially is actually chalcedony or agate that has been treated with heat and dye to achieve a uniform, deep color, since entirely natural black onyx is comparatively rare.