How Does Opal Get Its Color?

Individual opals are as unique as snowflakes. They can be light or dark, mostly blue or every color of the rainbow, vibrant with moving flashes of color or mostly static, transparent or translucent or opaque. What gives each opal its unique combination of these characteristics?

The Internal Structure of an Opal

Opals form when silica and water settle into cracks or voids in volcanic or sedimentary rocks. Over vast periods of time, the wet silica forms layers of silica spheres. Once the spheres reach a uniform size, opals begin to form. The spheres inside an opal are what cause the shifting colors for which opals are so well known.

Splitting Light Into Colors 

The orderly array of spheres within an opal diffract light in various ways, producing the unique range of color and movement that define opals. That color and movement (play-of-color) is a result of white light entering the opal and being diffracted into rays of its component colors. As those light rays exit the opal, they produce the play-of-color which makes opals so prized.

The color of opal can range across the full spectrum of light. They may consist of a single color or every color in the rainbow. Most opals flash at least two or three different colors. 

The Most Sought After Opals

The look of an opal varies depending on the background color. The background color (body color) of an opal is caused by tiny impurities suspended within the opal's spheres. It can range from clear to milky white to dark tones or black.

The background color, range of color and pattern, dominant hues, and intensity of color all affect an opal's desirability. The amount of play-of-color it displays is also very important in determining its value. The most desirable opals display play-of-color when viewed from any angle. The color itself may change with different viewing angles or light sources, but the amount and quality of color will remain the same.