Identifying a stone specimen is a foundational practice in geology, relying on observable characteristics and simple physical tests to accurately classify the material. Learning to identify stones allows collectors to understand the formation processes that created their specimen. A step-by-step approach helps gather the necessary data points to narrow down possibilities, moving from general observation to specific classification. This guide focuses on physical properties that can be tested with readily available tools, making mineralogy accessible.
Initial Visual Assessment
The first stage of identification involves a careful visual examination of the stone’s inherent properties. Color is often the most noticeable feature, but it is the least reliable property for identification because trace chemical impurities can drastically change the surface color. For example, quartz can appear clear, pink, or purple. Only some minerals, like malachite, have a consistent color that serves as a diagnostic trait.
Luster describes how a stone’s surface reflects light and is a more dependable visual characteristic. The two main categories are metallic, which shines like polished metal, and non-metallic, which includes several distinct appearances. Non-metallic lusters include vitreous (glassy), pearly (iridescent sheen), silky (fibrous appearance), and dull or earthy (no shine). Examining the specimen’s luster provides insight into its composition and crystal structure.
The crystal habit refers to the typical shape a mineral’s crystals assume when they grow freely. This structure can be described with terms like prismatic (elongated columns), micaceous (flaky sheets), or botryoidal (grape-like clusters). Many specimens display a massive habit, meaning the crystals are too small or intergrown to show an identifiable shape. Transparency is noted by determining if the stone is transparent, translucent, or opaque.
Essential Physical Testing
Active physical tests begin with the Mohs scale of hardness, which measures a mineral’s resistance to scratching. This qualitative scale ranges from 1 (talc) to 10 (diamond), based on the principle that a harder material scratches a softer one. Hardness can be estimated using common objects: a fingernail is about 2.5, a copper penny is 3.0, and a steel nail or knife blade is typically between 5.0 and 6.5. If a steel nail scratches the stone, the stone’s hardness is less than 5.5, which significantly narrows the possibilities.
The streak test is performed by scraping the stone across an unglazed porcelain plate to reveal the color of its powder. Unlike surface color, the streak color is consistently diagnostic because it is unaffected by impurities or weathering. For example, hematite can be black, silver, or reddish-brown in hand sample, but it always leaves a characteristic reddish-brown streak. Minerals harder than the streak plate (typically 6.5) will scratch the plate instead of leaving a streak.
Observing how a stone breaks distinguishes between cleavage and fracture, providing insight into its internal structure. Cleavage describes the tendency of a mineral to break along flat, parallel planes of weakness within its crystal lattice, resulting in smooth, reflective surfaces. Fracture refers to a break that occurs in any direction other than a cleavage plane, yielding an irregular surface. Conchoidal fracture is a common type, producing smooth, curved, shell-like breaks seen in materials like quartz or volcanic glass.
Specific gravity, or relative density, compares the density of the stone to the density of water. While precise measurement requires a scale, a simple field estimation is called “heft,” which compares how heavy the stone feels for its size. A stone with a high specific gravity, such as galena, will feel noticeably heavier than an equally sized piece of a lower-density mineral like quartz. Heft serves as a quick check, but the actual specific gravity value is a fixed, numerical property that is highly specific to a particular mineral.
Categorizing the Specimen
The final step is synthesizing the data to place the specimen into one of the three major rock classifications. Igneous rocks form from the cooling and solidification of molten rock (magma). Slow cooling underground (intrusive) results in large, visible, intergrown crystals. Rapid cooling on the surface (extrusive) results in a fine-grained, glassy, or vesicular texture with trapped air bubbles.
Sedimentary rocks are formed by the cementation of mineral fragments, organic matter, or chemical precipitates. These rocks are often characterized by distinct layering or bedding. The presence of cemented grains, such as sand or pebbles, or evidence of past life like fossils, strongly suggests a sedimentary origin. These rocks tend to be softer than many igneous rocks and may have a dull or earthy luster.
Metamorphic rocks are created when existing rocks are transformed by intense heat and pressure deep within the Earth. The most defining characteristic of many metamorphic stones is foliation, which presents as parallel alignment or banding of mineral grains. This layered appearance is a physical manifestation of the immense pressure the rock endured. A resulting high hardness helps distinguish a metamorphic specimen from the other two types.