What is the difference between black agate and black onyx?

Black agate and black onyx are both types of chalcedony, but they differ in their formation and appearance. Black agate typically has a banded structure, displaying layers of different shades of black or gray, while black onyx is characterized by its smooth, solid black color, often with white or lighter bands. Additionally, black agate is usually formed through volcanic processes, whereas black onyx is often created through sedimentation. Both stones are used in jewelry and decorative items, but their aesthetic qualities and origins set them apart.