Onyxes and opals are two of the most popular gemstones in the world. Onyxes are a type of quartz that is usually black or dark gray in color, while opals are a type of mineraloid composed of hydrated amorphous silica. Both stones have been used for centuries in jewelry and other decorative items. Onyxes are known for their strength and durability, while opals are prized for their unique play of color.