Opal October’s Birthstone
CHEMISTRY SiO2 . nH2O. Water usually 6 to 10% in precious opal, can be as high as 21%.
REFRACTIVE INDEX 1.44 – 1.47
HARDNESS 5.5 – 6.5
SPECIFIC GRAVITY 1.99 – 2.25
HEAT SENSITIVE Very
SPECIAL CARE INSTRUCTIONS Very heat senstive, clean with warm or room temperature soap and water. Avoid wearing gem where it will get rough treatment.
ENHANCEMENTS Impregnated with oil, wax, or plastic. Occasional. Smoked, to create black opal. Occassional. Treated with dye or chemicals to make light opal black. Occasional.
*Wearability is graded as Excellent, Very Good, Good, Poor, and Forget It! For more details see the article on “Hardness and Wearability.”
Opals stand in a class by themselves. More than any other gem, each opal is distinctly an individual. No other stone has as rich and varied folklore. They are both one of the luckiest and unluckiest gems a person can own. They are so unique, they have their own descriptive vocabulary. Opals are also the most delicate gems commonly worn. They require special care to insure their health and longevity.
The name evolved from the Greek “Opallus” which means to see a change in color. Later, the Latin word “opalus” came to mean precious stone.
There is some doubt that ancient authors were referring to the same stone we call opal today. Some scholars believe that many references are actually to iridescent gems, like iris agate.
Opal’s fire was long thought to be the result of iridescence. However, with the advent of scanning electron microscopes, we now know that it is a result of diffraction.
Opal is an amorphous form of silica, (SiO2.nH2O) chemically similar to quartz, (SiO2) but containing 3% to 21% water within the mineral structure. Gem grade opals are usually 6% to 10% water content.
Opal is a sedimentary stone. Under proper conditions, water percolates through the earth, becoming rich in dissolved silicates. When it enters a cavity, the silicates are deposited as tiny spheres. If they are uniform in size and shape, they will diffract light. If they are random in shape and arrangement, we have common opal.
Volcanic ash gives black opal its color, but inclusions have nothing to do with the play of color. That is due entirely to the tiny spheres. They must be smaller than 1500 angstroms for blue and violet colors, but no larger than 3500 angstroms to produce oranges and reds. To put that in perspective, 20,000 spheres are about the size of the period at the end of this sentence.
Opal grows by filling in cavities, regardless of their shape. Hence, we have many pseudomorphs, materials with shapes that are unrelated to the chemical content. The most common are opalized wood and seashells.
Opals are delicate, but well worth the care. Their most significant weakness has to do with the water content. If an opal is allowed to dry, it will crack and craze. In most cases, they do not need any special care while stored. However, if you live in a very dry climate, or keep them in a dehumidified room, some precautions are necessary. Keeping them in a tight plastic bag, with a damp piece of cotton or fabric will prevent dehydration.
Storing an opal in oil or glycerin is not recommended. It is unlikely to damage the opal, but it is unnecessary and requires tedious cleaning.
Because of their water content, opals are also highly sensitive to sudden changes in temperature. I know of a woman in Pennsylvania who wore a brooch on the outside of her coat. As she passed from the warmth of her house to the winter cold, there was an audible “crack” as her opal self-destructed.
Opals do not mind being hot or cold, it is the rate of change that damages them. You need to avoid situation like the one above, going from a warm house to the winter’s cold. Simply wearing an opal under clothing will protect them. Also, do not store opals near a heat source, an open window, etc., where they can be exposed to sudden temperature changes.
(Click Images below to see some more examples)
Being somewhat soft, they scratch easily. Realize that a large component of dust is quartz at 7 in hardness. At 5.5 to 6 in hardness, simply wiping the dust off an opal will gradually reduce its polish. The solution is to clean your opals using a soft cloth or brush, a mild detergent, and room temperature water. Then rinse the jewels to remove any residue. Clean doublets and triplets with the same method, but do not soak them. Soaking can dissolve the glue holding the layers together.
Prevention is the best solution to scratching and chipping. Opals are best suited for earrings, brooches, and pendants. These jewels receive little contact with harder objects, compared to what a finger ring experiences. If you do get an opal ring, choose a setting that protects it from coming in contact with other objects.
Understand that, if you wear an opal ring on a regular basis, it will require occasional repolishing. Reserving your ring for special occasions will greatly reduce the risk of damage.
Make sure you remove your ring before physical activities like gardening and sports. Also, do not immerse the gem in liquid chemical solutions, like dishwater. Opals are porous and absorb liquids.
Play of color, the phenomena of flashing or moving colors due to diffraction and not related to the body color.
Noble opal, or Precious opal. Opal with play of color.
Common opal or Potch, translucent to opaque opal without play of color.
Fire in opal refers to play of color.
Fire opal is named for its body color and may or may not have play of color.
Crazing, cracks that develop as an opal dehydrates.
Black opal, precious opal with black body color. Also used for black potch covered with thin layer of crystal opal that lets the black under layer show through.
Semiblack, or gray opal, precious opal with dark body color.
Light opal is in between semi-black and white.
White opal, precious opal with white or very light body color.
Crystal opal, transparent to semitransparent, colorless body with play of color.
Doublets and Triplets make use of opal that is too thin to use as a solid gemstone. A doublet is a thin layer of precious opal glued to a black base. A triplet adds a transparent, quartz cap.
Purists prefer the base material to be common opal. However many black materials are used, including old phonograph records.
Black crystal opal, transparent to semitransparent opal, with dark body color and play of color.
Fire opal, translucent to transparent, with yellow, orange, or red body color. May or may not have play of color. Also called Mexican opal or Sun opal.
Boulder opal, a thin seam of precious opal on ironstone matrix. Since this is a natural occurrence, its value is higher than that of a man made doublet.
The ironstone is very dark, which makes the fire stand out and gives a close resemblance to black opal.
Matrix opal consists of thin seams or spots of precious opal in matrix. The matrix is usually dark ironstone, but occasionally a light sand stone. Also called Mass opal.
Many thanks to Dr. Joel Arem, for allowing IGS to use some of the Opal images from his Color Encyclopedia of Gemstones.