Opal is a unique gemstone, captivating observers with its internal flash of spectral colors known as play-of-color. This optical effect is caused by the diffraction of light through a precise, ordered structure of microscopic silica spheres within the stone. While all precious opals share this phenomenon, the specific appearance and classification of an opal are determined by its body tone, which is the underlying background color. Among the various types, the crystal opal stands out, defined by a fundamental characteristic related to light transmission.
Defining Characteristics of Crystal Opal
The term “crystal” in crystal opal refers directly to the stone’s diaphaneity, or its ability to transmit light. This opal type is defined by having a body that is translucent or semi-transparent. Unlike opaque opals that completely block light, a good quality crystal opal will appear glass-like, often allowing a distorted image or light source to be seen through the gem. This transparency is the single most important factor for its classification.
The body tone of a crystal opal can vary widely, ranging from dark gray to nearly colorless. Gemological grading systems use the “N” scale for body tone, where crystal opals typically fall into the lighter range (N7 to N9). When light enters a transparent body, the play-of-color is given a unique three-dimensional depth, often appearing to float within the stone. This glass-like clarity allows for a distinct visual experience compared to opals contained within an opaque matrix.
Distinguishing Crystal Opal from Other Opal Types
Distinguishing crystal opal from other varieties relies on assessing its degree of transparency and the underlying body tone. The most commonly confused types are Black Opal and White Opal. Black Opal is characterized by a dark, opaque body tone (N1 to N4). This dark background creates a high-contrast canvas that makes the play-of-color appear exceptionally vibrant.
White Opal, often called light opal, is defined by its light, opaque body tone (N7 to N9). While it shares a similar light body tone range with some crystal opals, the white opal’s body is milky or opaque, preventing light from passing through it. The essential difference is that the crystal opal’s body is clear or translucent, whereas the white opal’s body is cloudy or milky.
When classifying an opal, gemologists prioritize the body tone and transparency over the color play itself. A crystal opal can possess a dark body tone, referred to as a “black crystal opal,” but its translucent nature still keeps it distinct from an opaque Black Opal. The ability to see into the stone is the physical marker that separates the crystal variety from its opaque counterparts.
Evaluating Quality and Value
The value of a crystal opal is determined by several factors, with the overall brightness of the play-of-color being the most significant. Brightness is graded on a scale, with the most intense and vivid colors receiving the highest ratings, such as B5. A small opal with vivid color saturation can often be more valuable than a larger stone that exhibits only a dull play-of-color. The thickness of the color bar, or the layer of precious opal, is also considered, as a thicker bar suggests a more durable stone and can contribute to the depth of the color play.
The pattern of the color flashes is another major metric for assessment. Patterns are formed by the specific arrangement of the internal silica spheres and include:
- Pinfire, which are tiny specks of color.
- Rolling flash.
- The highly coveted harlequin pattern, which features distinct, blocky patches of color.
Because of its transparent nature, the color of a crystal opal is heavily influenced by the material placed behind it. When set over a dark background, such as black jewelry metal or a dark backing material, the translucent body allows the dark color to show through. This dramatically enhances the contrast and perceived brightness of the play-of-color. This characteristic is often used by jewelers to maximize the visual impact of the stone.