Common opal is a form of silica that lacks the vibrant display of colors seen in precious opal. This mineral is found worldwide, and understanding its physical attributes is key for identification by enthusiasts and collectors.
Understanding Common Opal’s Characteristics
Common opal presents a diverse array of colors, frequently appearing in opaque to translucent pastel shades. These typically include earthy tones such as white, yellow, red, brown, green, and blue. Unlike precious opal, it does not exhibit the “play of color,” which is the iridescent display of spectral hues.
The luster of common opal ranges from waxy to resinous, giving its surface a somewhat dull or plastic-like appearance. This sheen helps distinguish it from minerals with a glassy or metallic luster. Its transparency varies from completely opaque, where no light passes through, to translucent, allowing some light to diffuse.
Common opal is relatively soft, registering between 5.5 and 6.5 on the Mohs hardness scale. It can be scratched by harder materials like a steel file or a quartz crystal. Testing its hardness provides an initial indication of its identity.
The specific gravity of common opal is generally lower than many other minerals, typically ranging from 1.9 to 2.2. This lower density means it feels relatively lightweight for its size compared to denser stones like quartz. This difference in weight is a helpful, though not definitive, identification clue.
When common opal breaks, it typically exhibits a conchoidal fracture pattern, displaying smooth, curved, shell-like indentations. This distinctive fracture results from its amorphous structure, which lacks the organized internal crystalline arrangement found in many other minerals. The streak of common opal, the color of its powder when rubbed against an unglazed porcelain plate, is consistently white.
Avoiding Misidentification
Distinguishing common opal from minerals like quartz or chalcedony requires careful observation. While quartz and chalcedony can occur in similar colors and sometimes have a waxy appearance, they are significantly harder than common opal, typically rating 7 on the Mohs scale. Common opal’s amorphous structure, lacking a defined crystalline form, also sets it apart from the crystalline nature of quartz and chalcedony.
Common opal can sometimes be confused with glass due to shared characteristics like conchoidal fracture. However, natural common opal will not exhibit features found in manufactured glass, such as internal flow lines or trapped air bubbles. The presence of these imperfections typically indicates a man-made material.
Accurate identification of common opal relies on evaluating a combination of its physical characteristics rather than any single trait. Relying on multiple tests, such as assessing its specific gravity, hardness, luster, and fracture pattern, provides a more comprehensive and reliable assessment. This approach ensures correct identification and avoids confusion with similar substances.