A raw gemstone is a mineral specimen found in its natural, unprocessed state, meaning it is uncut, unpolished, and unfaceted. Identifying these rough stones without professional gemological equipment presents a unique challenge, as the vibrant colors and clarity of finished gems are not yet visible. This preliminary identification process relies on a sequence of straightforward physical and visual field tests that, when combined, narrow down the possibilities. This guide provides practical steps for the general enthusiast to make an educated assessment of an unknown raw gemstone.
Initial Visual Clues
The first step in identification involves a careful visual examination of the stone’s surface characteristics. Color is the most immediate trait, but it is often misleading because trace impurities can drastically change a mineral’s hue. Stones like quartz can appear in a wide range of colors due to these foreign elements and are known as allochromatic. Conversely, idiochromatic gems, such as peridot, owe their color to elements that are an essential part of their chemical composition, giving them a more consistent color.
A stone’s luster describes the way light reflects off its surface, offering a more stable clue than color. Common descriptions range from vitreous (resembling glass) to silky (caused by fine parallel fibers). Other appearances include metallic or dull, which indicates a non-reflective, earthy surface. Observing the quality of this reflection gives an initial indication of the stone’s internal structure and composition.
The natural growth shape, or crystal habit, provides another important visual clue about the internal atomic arrangement. Crystals growing freely often develop distinct shapes, such as the six-sided prisms of quartz or the hexagonal structure of corundum. Other minerals may form in a massive habit, appearing as an irregular lump without any flat faces. Recognizing these characteristic shapes can significantly limit the number of potential mineral species.
Testing Physical Resistance
Once the visual properties are noted, the next step involves testing the stone’s resistance to mechanical force. The Mohs Hardness Scale measures a mineral’s relative resistance to scratching, ranking materials from 1 (softest, like talc) to 10 (hardest, like diamond). This test is performed by attempting to scratch the unknown stone with a material of known hardness. For field identification, common objects serve as reference points, such as a fingernail at 2.5, a copper penny at about 3.5, or a steel knife blade at approximately 5.5.
To perform a scratch test correctly, hold the specimen firmly and press the point of the reference material against a clean surface. Dragging the point across the stone requires firm pressure, and any apparent mark should be wiped away to confirm it is an etched line and not just powdered residue. If the known material leaves a permanent scratch, the gemstone is softer; if the gemstone scratches the known material, its hardness is greater. A stone that can be scratched by a steel nail but not a copper penny, for example, has a Mohs hardness between 3.5 and 5.5.
A further test of structural integrity is observing how the stone breaks, which reveals whether it exhibits cleavage or fracture. Cleavage is the tendency of a mineral to break along flat, predictable planes of structural weakness, specific to the mineral’s internal atomic arrangement. Fracture, by contrast, is an irregular breakage that occurs in any direction not related to a plane of weakness. The most recognizable type of fracture is conchoidal, which results in smooth, curved, shell-like surfaces often seen in glass or pure quartz.
Analyzing Specific Material Properties
Moving beyond external observation and resistance, testing specific material properties provides more definitive, chemistry-based evidence for identification. The streak test determines the color of the mineral when it is reduced to a fine powder, which can be a more reliable indicator than the stone’s body color. This is done by scraping the gemstone across a piece of unglazed porcelain, commonly called a streak plate, to leave a trail of powder.
The color of this powder, the streak, often remains consistent even if the stone’s external color varies due to impurities. For instance, the mineral hematite may appear black or silver, but it consistently leaves a reddish-brown streak. Streak plates have an average hardness of about 6.5 to 7 on the Mohs scale, meaning that any gemstone harder than this, such as corundum, will simply scratch the plate without leaving a powder streak. For these harder materials, the streak is described as white or colorless, and the test is not useful.
The final property to consider is density, which is measured by specific gravity (SG), a ratio comparing the stone’s weight to the weight of an equal volume of water. Unlike size, a gemstone’s specific gravity is a precise value that is consistent for each mineral species. Since a professional hydrostatic balance is typically unavailable, a practical estimation is achieved through the “heft” test, a subjective assessment of how heavy the stone feels relative to its size.
A more accurate, though still rough, estimation can be made using the principle of water displacement. By weighing the stone in the air and then weighing it while submerged in water, the difference in weight can be used to calculate a rough specific gravity value. This ratio allows for the distinction between two visually similar gems, such as a heavy garnet and a lighter quartz of the same size. Combining the results from all these tests provides the necessary data to narrow the possibilities and identify a raw gemstone.