Opals display a phenomenon known as play-of-color. When an Opal has play-of-color, it is categorized as precious opal. The main categories of precious opal are white, black, boulder, and crystal or water. Fire opal, also known as Mexican Opal, sometimes does not show play-of-color.
How Opals Formed
Opal is the result of seasonal rains that drenched dry ground in regions such as Australia’s semi-desert “outback.” The water soaked deep into underground rock, carrying dissolved silica (a compound of silicon and oxygen) downward.
During dry periods, much of the water evaporated, leaving solid deposits of silica in the cracks and between the layers of underground sedimentary rock. The silica deposits formed opal.
Play of Colors
Play-of-color occurs in precious opal because it’s made up of sub-microscopic spheres stacked in a grid-like pattern—like layers of Ping-Pong balls in a box. As the lightwaves travel between the spheres, the waves diffract, or bend. As they bend, they break up into the colors of the rainbow, called spectral colors. Play-of-color is the result.
Opals have 2 types: precious and common. Precious Opals are opals that have play-of-colors. Common Opals do not have play-of-color. Common Opals are opaque stones.
Opals are often divided into types based on background color. Even though there are many different categories for opals, here are the main five types:
All other quality factors being equal, many buyers favor the dark background color of black opal. This is partly because play-of-color tends to stand out attractively against a dark background. The contrast of play-of-color to bodycolor makes black opals very popular. Additionally, black opals are considered to be the rarest (white opals are more common).
An opal with a translucent to opaque white and other light color backgrounds (bodycolor) with play-of-color is called white opal.
Fire opal is transparent to translucent with a bodycolor that is usually yellow, orange or red. This material, which might show play-of-color, is also known in the trade as “Mexican opal” or “Mexican fire opal”.
Boulder opal is translucent to opaque opal with play-of-color within a host rock. Thin layers of opal exist within the host rock (called matrix). The opal is cut with the matrix attached and is part of the finished gem.
Crystal and Water Opal
Crystal opal is transparent to semitransparent with a clear background. This type of opal can show excellent play-of-color. Water opal might or might not display play-of-color. If it does show play-of-color, it is faint and covers only small portions of the gem.
With an opal, clarity is its degree of transparency and freedom from inclusions. An opal’s clarity can range all the way from completely transparent to opaque.
A cloudy or milky background color can sometimes signal a lack of stability. Opals can have fractures and surface blemishes. Matrix, or host rock, along with signs of crazing, a fine network of cracks, have an impact on opal’s durability and value.
Opals can be treated by impregnation with oil, wax or plastic. Opal doublets or triplets are thin slices of opal glued to a base material and covered with a thin dome of clear quartz. These gems are more resistant to scratching but are considered less valuable. Some opal can be dyed.
Care and Cleaning
The only safe way to clean opal is with warm, soapy water. Opals are sensitive to heat or excessive dryness which can lead to crazing or internal cracks.