Black opal is a highly valued gemstone known for its dramatic play-of-color against a dark background. This contrast makes the internal flashes of color, known as the “fire,” appear more vivid and intense than in lighter opals. The rarity of genuine black opal, particularly those sourced from Australia, has led to the creation of many treated, synthetic, and composite stones that imitate its appearance. Understanding the characteristics of natural black opal and the methods used to create fakes is necessary to ensure the stone you purchase is authentic.
Defining Natural Black Opal
The body tone, not the surface colors of the fire, is the defining characteristic separating genuine black opal from other types. This background color is officially graded on a scale from N1 (jet black) to N4 (dark gray) using the Australian Opal Association’s body tone chart. Opals with lighter body tones, such as N5 or N6, are classified as dark opals, while N7 to N9 are considered light opals.
This dark body tone results from the geological environment where the opal forms. In Lightning Ridge, Australia, the primary source for black opal, the precious opal often forms on a layer of dark common opal, known as “potch.” This dark base sits directly beneath the thin layer of precious opal, creating a deep backdrop that amplifies the visible colors. A true, solid black opal is a single piece of natural material where the dark potch naturally transitions into the color-producing layer above it.
Treatments That Mimic Black Opal
Many natural opals that are not dark enough to be classified as black opal are chemically or thermally altered to imitate the dark body tone. These processes use natural, low-value opal material, such as porous matrix opal or crystal opal, and introduce a darkening agent. The resulting stone is still a natural opal, but its color is man-made and must be disclosed to the buyer.
One common method is the sugar/acid treatment, often applied to porous matrix opal from Andamooka, Australia. The opal is first soaked in a hot sugar solution, and then immersed in concentrated sulfuric acid. This chemical reaction causes the sugar to carbonize, depositing tiny black carbon particles into the microscopic pores of the opal. Under magnification, this carbonization can be observed as a dense layer of minute black spots or patches, rather than the uniformly dark, non-particulate background of a natural stone.
Another technique is the smoking or heat treatment, which is frequently used on porous hydrophane opals, like those from Ethiopia. The opal is wrapped and exposed to smoke from a smoldering source, allowing fine soot particles to penetrate the stone’s porous structure. This process darkens the body tone by embedding carbon, creating the illusion of a natural black opal. These treated stones can sometimes show uneven smoke distribution or a change in color if they become wet, offering clues to their alteration.
Synthetic and Composite Black Opals
Beyond treating natural material, the market includes completely man-made or assembled stones designed to look like black opal. Synthetic opals, such as those pioneered by Pierre Gilson, share the same chemical composition as natural opal, being composed of silica spheres, but they are grown in a laboratory over a short period. The internal structure of these synthetics is often too regular, displaying a characteristic “lizard skin” or “chicken wire” pattern under magnification.
A sign of a synthetic opal is the presence of an unnatural, columnar structure when viewed from the side. This results from the rapid, uniform sedimentation process in the lab, a feature not seen in natural opal. The play-of-color in synthetics tends to appear two-dimensional and static, lacking the dynamic depth and movement seen when a natural stone is rotated.
Composite stones, known as doublets and triplets, use a thin slice of genuine or synthetic opal to create the appearance of a larger, darker stone. An opal doublet consists of two layers: a thin slice of light opal glued to a dark backing, such as black potch, ironstone, or industrial glass. An opal triplet adds a third layer: a clear dome of plastic, glass, or quartz placed over the opal slice, which magnifies the color and protects the thin layer.
Practical Inspection Methods for Buyers
The most reliable way to identify a composite stone is by inspecting the side or girdle of the piece. A doublet or triplet will exhibit a perfectly straight line where the thin opal layer is adhered to the dark backing, a sharp delineation that does not occur in a natural, solid opal. If the stone is unset, the back of a composite may appear flat and uniform, sometimes with a plastic-like or glassy sheen, unlike the organic, sometimes irregular, nature of a solid opal’s potch backing.
To check for sugar/acid treatment, examine the stone’s surface under 10x magnification. Look for a granular appearance or tiny black spots scattered throughout the material, which are signs of carbon particles deposited in the pores. A natural black opal’s body tone is a result of its bulk material, not surface-level carbonization.
A simple translucency test can reveal an altered stone. Genuine black opal is generally opaque, meaning little light passes through the body of the stone. If a strong white light is shone through the back or side and the entire material is easily transparent, it may indicate a doublet with a thin backing or a heavily treated, porous crystal opal.