Jasper is a dense, opaque variety of microcrystalline quartz (\(\text{SiO}_2\)). It is classified as an impure form of chalcedony, often containing up to 20% foreign materials like clay, hematite, or goethite. Collectors rely on a combination of visual cues and simple physical properties to reliably distinguish this material from its many lookalikes.
Visual Identification: Color, Pattern, and Texture
Jasper is recognized for its earthy color palette, which includes deep reds, yellows, browns, and greens; blue is the rarest shade. These vibrant colors are caused by minute mineral inclusions trapped during formation. For instance, the common red variety is due to iron(III) oxides (hematite). Shades of yellow or brown often result from iron hydroxides like limonite.
The patterns within jasper are a defining feature, giving rise to varietal names like “Picture Jasper” or “Ocean Jasper.” These patterns manifest as distinct bands, swirls, or spots, created by the consolidation of silica-rich sediment or volcanic ash. Orbicular jasper displays concentric rings or eye-like formations, while Picture Jasper often features intricate, landscape-like scenes. A primary visual characteristic is its opacity; genuine jasper will not allow light to pass through, distinguishing it from translucent chalcedony forms like agate. Polished jasper typically exhibits a luster ranging from a waxy sheen to a vitreous, glass-like appearance.
Practical Physical Testing for Confirmation
Confirmation relies on physical testing, beginning with the Mohs hardness scale. Jasper consistently measures between 6.5 and 7 on this scale, which is relatively high. This hardness means that jasper can easily scratch a piece of standard glass, which has a hardness of approximately 5.5. A steel nail, which is generally 6.5 or less, cannot scratch the jasper. A simple scratch test against a known softer material can thus quickly eliminate many potential lookalikes.
Another reliable test is the streak test, which involves rubbing the specimen across an unglazed porcelain plate. Despite its wide range of body colors, jasper will always produce a white or colorless streak. This result is a direct consequence of its primary component being microcrystalline quartz. When broken, jasper exhibits a conchoidal fracture pattern, meaning the surface of the break is smooth and curved, resembling the inside of a clam shell.
Specific gravity testing provides secondary confirmation, although it is less practical for field use. Jasper’s specific gravity falls in the range of 2.5 to 2.9 \(\text{g}/\text{cm}^3\). This narrow density range results from the dense interlocking of its microscopic quartz crystals and mineral impurities.
Differentiating Jasper from Common Lookalikes
A common identification challenge is distinguishing jasper from chert or flint. While jasper is essentially a colorful, impurity-rich variety of chert, chert and flint typically lack the vibrant colors and complex patterns characteristic of true jasper. Chert generally presents in drab shades of gray, white, or brown, without the vivid iron-oxide induced hues of red or yellow seen in jasper.
Jasper also requires differentiation from common opal, which can appear similarly opaque and patterned. The primary distinction lies in composition: opal contains a significant amount of water (up to 20%), making it considerably softer than jasper. Common opal usually measures 5.5 to 6.5 on the Mohs scale, meaning it is noticeably softer and will not scratch glass as readily as jasper.
Jasper is occasionally confused with petrified wood, especially pieces where the original wood structure has been completely silicified. However, petrified wood often retains the subtle structural patterns of the original wood grain, which are absent in the purely sedimentary or volcanic formations of jasper. Focusing on the definitive hardness, the white streak, and the degree of color saturation provides the clearest path to correct identification.