Opal is made of the same ingredients as quartz, except it contains a little water and has not been compressed into crystals. As a result, it is softer than quartz and has to be treated a little more carefully to avoid damage.

The shifting colors seen in opal, called "fire," are the result of microscopic spherical structures within the stone which reflect different wavelengths of light depending on their spacing, creating the colorful shimmering effect.

There is no other gemstone that looks remotely like it. It comes in both black and white varieties, with black being the most rare. Australia is the principal source of opal today. Like other non-transparent stones, it is usually cut into a smooth, rounded, polished dome called a cabochon.

Some legends say that opal is good for the eyes, both improving vision and warding off eye troubles. It also has a reputation for sharpening the mind and the emotions. Opal is a symbol of fidelity, but it came with a price, since it would bring trouble to someone who was unfaithful.

Opal is October's birthstone.