There is no other gemstone that looks remotely like iridescent opal. 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. Opal is made of the same ingredients as quartz, except it contains a little water and has not been compressed into crystal form. As a result, it is softer than quartz and has to be treated a little more carefully to avoid damage. It comes in both black and white varieties, with black being the rarer. Australia is the principal source of opal today. Like other non-transparent stones, it is usually smooth-cut into 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, and was thought to bring trouble to someone who was unfaithful.
Opal is October's birthstone. It has a Mohs hardness rating of 6. Opal should be treated with some care, and should never be placed in an ultrasonic cleaner, as it can cause the stone to crack.