Opal is a mineraloid composed of hydrated silica (silicon dioxide containing water, typically 3 to 10 percent by weight). Unlike true minerals, it is amorphous, lacking a defined crystal lattice. It possesses a highly ordered internal structure: a three-dimensional lattice of microscopic, nearly uniform silica spheres.
When light enters an opal with this specific structure, it encounters the regular arrangement of the silica spheres and the tiny gaps between them. This interaction causes the light to diffract and interfere, splitting white light into its component spectral colors. The resulting display of shifting, flashing color is known as “play-of-color,” and it is the primary characteristic used to classify valuable opals.
Precious Versus Common Opals
The fundamental distinction in opal classification is based on the presence or absence of the play-of-color phenomenon. Precious opals exhibit this brilliant, shifting display of spectral hues, resulting from the highly ordered stacking of internal silica spheres that diffract light.
Common opal, often referred to as “potch,” does not display play-of-color. Its silica spheres are either varied in size or arranged randomly, preventing the necessary light diffraction. Potch is far more abundant globally than precious opal and is generally opaque or translucent with a milky, white, gray, or brown body color.
Opals Classified by Body Tone
The body tone refers to the background color of the opal, acting as the canvas against which the spectral flashes are displayed. This background color is graded on a scale from N1 (jet black) to N9 (white) and is a primary determinant of value. Darker body tones lead to higher valuations because the play-of-color stands out more vividly.
Black Opal
Black Opal is the most valuable variety, distinguished by a body tone ranging from dark gray to jet black (N1 to N4). The dark background provides the highest contrast, making the internal color flashes appear exceptionally brilliant. True black opal is rare.
White or Light Opal
White or Light Opal has a body tone ranging from white to light gray or cream (N7 to N9). While these opals exhibit play-of-color, the lighter background means the spectral hues do not contrast as strongly as in black opals. White opals are common and are generally the most affordable precious opals.
Crystal Opal
Crystal Opal is defined by its transparency or semi-transparency, allowing light to pass through the stone. This type can occur with any body tone. When held up to a light source, a distorted image can be seen through the stone, giving it a glassy appearance.
Opals Classified by Structure and Host Rock
Some classifications are based on how the opal integrates with the host rock it formed within. These structural types, primarily found in Queensland, Australia, are defined by the permanent presence of the surrounding geological material. This distinguishes them from opals cut as solid pieces of pure silica.
Boulder Opal
Boulder Opal is characterized by a natural layer of ironstone or sandstone host rock permanently attached to the back of the precious opal layer. The opal forms in thin seams or veins within these ironstone boulders. Leaving the dark host rock intact provides a natural backing that enhances the play-of-color, mimicking the high contrast of black opal.
Matrix Opal
Matrix Opal is a formation where small patches or veins of precious opal are distributed throughout a porous host rock. The opal is intertwined with the ironstone or sandstone, creating a speckled or web-like pattern. This structure means the entire piece is a blend of host rock and precious silica.
Understanding Enhanced and Synthetic Opals
Not all opals available commercially are purely natural, solid stones, requiring consumers to be aware of lab-created and treated varieties.
Synthetic Opals
Synthetic opals, such as those made through the Gilson or Kyocera processes, are manufactured in a laboratory to replicate the chemical composition and structure of natural opal. These lab-grown versions typically display a highly uniform, often “lizard-skin” or “snake-skin” pattern of color. When viewed from the side, they may show a tell-tale columnar structure that is not present in natural stone.
Treated or Enhanced Opals
Treated or enhanced opals are natural stones artificially altered to improve their appearance, usually by darkening the body tone to imitate the value of black opal.
A common technique involves a sugar and acid treatment, where the opal is soaked in a sugar solution and then carbonized with acid, creating dark carbon particles within the pores. Another method is smoke treatment, often applied to porous Ethiopian opals, where the stone is heated to allow smoke to penetrate and darken the body. Ethical sellers must disclose these treatments, as they significantly affect the stone’s market value.