The image of a geologist in the field, hammer in hand, occasionally pausing to lick a rock, is a persistent and often humorous stereotype. This practice is not a bizarre ritual but an old, low-tech field method used for mineral identification. While wetting a rock’s surface is effective, modern geologists have largely changed the practice due to safety concerns. This article explores the practical reasons for using moisture and the other simple techniques geologists use to understand Earth materials.
Why Moisture Aids Mineral Identification
The primary reason for applying moisture to a rock specimen is to enhance the visibility of its surface features, which are often obscured when dry. A thin layer of water acts like a magnifying lens, increasing the contrast between different mineral grains and revealing micro-textures. This effect is important because many dry rocks appear dull, dusty, or uniformly colored, making fine details difficult to see with the naked eye or a hand lens.
Wetting the surface dramatically improves the mineral’s luster, which describes how light reflects from the crystal surface. A wet surface reveals the true crystalline structure, making it easier to distinguish between minerals with metallic, glassy, or earthy lusters. Moisture is also helpful for identifying fine-grained sedimentary rocks, such as shale or siltstone, or for distinguishing clay minerals. For instance, clay minerals like kaolinite feel characteristically sticky when wet, and a true fossil bone will adhere to the tongue due to its porous structure, unlike a regular rock.
The Reality of the Practice Today
While wetting a rock remains a useful technique, the act of licking has largely been replaced by safer alternatives in modern geological practice. Historically, the tongue was the most convenient and readily available source of moisture in the field. Today, however, geologists are aware of the hygiene risks associated with direct contact, as rocks can harbor pathogens like Salmonella or E. coli.
The more significant concern is the potential toxicity of certain minerals, which can pose a serious health risk even in small quantities. Minerals containing toxic elements like arsenic, lead, or mercury should never be tasted. For example, cinnabar (a mercury ore) and galena (a lead ore) are common minerals that are potentially dangerous if ingested. Consequently, most field geologists now carry a small water flask or a spray bottle to safely moisten a specimen without the risk of contamination or poisoning.
Essential Field Identification Techniques
Moisture application is only one tool in the geologist’s field kit, which is dominated by quick, low-tech tests. One of the most reliable methods is the streak test, which involves scraping the mineral across an unglazed porcelain plate. This technique reveals the color of the mineral’s powdered form, a property that is more consistent and diagnostic than the often-variable surface color.
Another foundational test is determining the mineral’s hardness using the Mohs scale, which ranks resistance to scratching from 1 (talc) to 10 (diamond). Geologists use common objects like a fingernail, a copper penny, or a steel knife to estimate a mineral’s position on this scale. For identifying carbonate minerals like calcite, a geologist may apply a drop of dilute hydrochloric acid. A visible effervescence or bubbling reaction confirms the presence of carbonate, as the acid releases carbon dioxide gas.