Webster’s defines opal as “…a mineral….that is a hydrated amorphous silica softer and less dense than quartz and typically with definite and often marked iridescent play of colors.” The opal is a stone so distinctive that everyone can identify it, with its many colored flashing lights. The brilliance and vibrant colors within an opal resemble the colors of fall, so it is an appropriate birthstone for the month of October. Opal is unique among gems, as it displays an array of very brilliant miniature rainbow iridescent effects, all mixed together. These colors were very highly prized in Roman jewelry.
The play of color comes from opal’s formation process, which is different than many gems. The color comes from the reflection of the scattering of light from the minute, uniformly sized and closely packed silica spheres that make up precious opal. The arrangement of these spheres, which vary in size and pattern, is responsible for the different colors. The more brilliant the color, or fire, the more valuable the gem. The most familiar opals are nearly opaque white or more translucent white, some having a black or reddish background. All opals vary greatly in their color of fire. Some have only red and orange lights, some also have green, and some also have yellow and blue lights. Black opals may have all of these colors as well as purple. Opals show just about every shade of every color in a variety of combinations. Opals with an abundance of red are usually the most expensive. Those strong in blue and green are equally as beautiful, but less rare, so their price is somewhat less. One of the most rare opals is called the Harlequin opal, which displays color patterns resembling a checkerboard.
Opal is a delicate and soft stone, rating a 5.5 to 6.5 on the hardness scale. It is usually milky and translucent. Opal is a hardened silica gel containing 5-20% water. Some opals may crack if allowed to dry out too rapidly after being mined. Opals may be somewhat porous, in which case it is dangerous to immerse it in liquids other than water. Opal is amorphous, meaning, it has no crystal structure. The only other major amorphous gemstone is amber. Good quality opals are transparent, not milky.
Imitation opals have been made using Slocum stone, a man-made glass that gives a play of color. Chips of opal and colored plastic are also put into hollowed rock crystal, and an imitation opal from Gilson Laboratories uses silica spheres.
Opal is formed when silica was liquefied and washed down into fissures in the surrounding rock, where it then solidified into a hardened gel. Unlike most other gemstones, opal is therefore not a crystal, but rather an amorphous solid. Opal is found in fossilized shell, wood and bone. Some precious opal forms in gas cavities in volcanic rocks, as in Mexico, and Slovakia, but most Australian deposits occur in sedimentary rocks. It is not found in many ancient archaeological diggings for a good reason, as the stones do not last for thousands of years. They contain 5-20% water, which dries out over the years, causing them to become brittle and lose their hardness. The fire, or color, in opals is made by the cracks in it, thus its beauty is also one of its weaknesses, though the cracks are not visible to the naked eye. If the cracks are visible without a magnifying lens, it is not considered to be a fine opal.
The most valuable opals come from Queensland and New South Wales in Australia, which have been the main producer of opals since their discovery in the 1870’s. Nowadays, opals are mostly found in Australia, Mexico, the USA, South America, Britain, Canada, Brazil, Czech Republic and Slovakia.
Your opal will last a long time if you take good care of it. You should treat your opal with some care to prevent any scratches or blows. The stones should never be kept in oil or any other chemicals. This may cause them to lose some or all of their fire. Because opals contain some water (as much as 20% water), they should never be stored in a bank or vault for long periods of time because of the dehumidifiers used in many vaults. If opals get too dry, they tend to crack. This phenomenon is called “Crazing,” and it wipes out the value of the stone. You should avoid leaving your opal near anything potentially drying. To keep your opal from drying, it may be helpful to immerse it in water for several hours from time to time.