Rough sapphire is the uncut form of the gemstone, sought by prospectors and collectors. This material is a variety of the mineral corundum, an aluminum oxide crystal (\(\text{Al}_{2}\text{O}_{3}\)) formed under intense geological pressure and heat. Identification relies on understanding the unique physical and optical properties of corundum, which distinguish it from common stream pebbles and other minerals. Unlike a finished, faceted gemstone, rough sapphire retains its original, often irregular shape and surface texture. Identifying rough sapphire involves a systematic process, starting with visual inspection and moving toward definitive physical testing.
Recognizing the Crystal Structure and Luster
The first step in identification is a thorough visual inspection, concentrating on the specimen’s shape and surface qualities. Corundum belongs to the trigonal crystal system, which often manifests externally in a characteristic hexagonal or six-sided prismatic shape. Rough crystals are frequently found as distinct barrel-shaped forms or tapering hexagonal pyramids, even if they are water-worn.
Examining the surface closely, perhaps with a magnifying loupe, may reveal fine parallel lines known as striations or growth lines. These lines run perpendicular to the crystal’s main axis, tracing the hexagonal outline. Rough sapphire typically displays a vitreous, or glass-like, luster, which can appear sub-adamantine on fresh breaks.
Color distribution within the stone is another important visual cue, as natural sapphire often exhibits color zoning. This phenomenon occurs when trace elements, such as iron and titanium, are unevenly incorporated into the crystal lattice during growth, causing variations in hue. A natural sapphire might display bands of darker and lighter color, or subtle patches of different shades. Transparency also varies greatly, ranging from opaque to highly transparent, depending on the number of internal inclusions present.
Testing for Hardness and Density
Once a specimen has passed the initial visual checks, physical tests for hardness and density are performed, as these properties are highly diagnostic for corundum. Hardness is measured on the Mohs scale, which ranks minerals based on their resistance to scratching. Sapphire ranks at 9 on this scale, making it the hardest natural mineral after diamond.
This extreme hardness is a powerful distinguishing characteristic; a rough sapphire should be able to scratch materials with a lower Mohs rating, but it should remain unscratched itself. For a layperson, a practical test is to attempt to scratch the specimen using a known piece of quartz (Mohs 7). If the quartz leaves a groove on the specimen, the material is not sapphire. A sapphire specimen can also be used to test the hardness of other common materials, such as a steel file (Mohs 6.5) or ordinary glass (Mohs 5.5).
Density, or specific gravity (SG), provides another definitive measure, as it compares the stone’s weight to the weight of an equal volume of water. Corundum has a consistent specific gravity range of 3.98 to 4.06. This value is significantly higher than that of many other common minerals, meaning that a rough sapphire will feel noticeably heavy for its size.
To estimate this density in the field, one can perform a simple “heft” test, comparing the weight against a known material of similar size. For a more accurate reading, a digital scale and a small container of water can be used to perform an approximate displacement test. By weighing the stone in air and then weighing it while suspended in water, the specific gravity can be calculated, which should fall within the 3.98 to 4.06 range.
Ruling Out Common Mimics
Several common minerals and materials can be mistaken for rough sapphire, but the distinct properties of corundum allow for differentiation. Quartz is perhaps the most frequent mimic, as it is abundant and comes in various colors, but its crystal structure is different and its Mohs hardness is only 7. A rough quartz crystal will typically display a prismatic habit with prominent horizontal growth lines, differing from the often barrel-shaped or pyramidal form of sapphire.
Another common confusion is with topaz, which ranks lower on the Mohs scale at 8. While topaz is relatively hard, it will not be able to scratch corundum, and a piece of sapphire will easily mark the topaz. Furthermore, the specific gravity of topaz (around 3.5 to 3.6) is noticeably lighter than sapphire’s density of nearly 4.0.
Spinel is another mineral that can look visually similar, particularly in its rough octahedral or dodecahedral crystal forms. However, the specific gravity of spinel is lower, generally falling between 3.5 and 3.7. Unlike sapphire, which is double refractive, spinel is singly refractive, an optical property that results from its different internal structure.
Finally, manufactured glass cullet is occasionally mistaken for rough gems, but it is easily ruled out by the previously discussed tests. Glass lacks any characteristic crystal structure, may show telltale signs like internal bubbles or swirl marks, and is much softer and lighter. With a Mohs hardness of about 5.5 and a specific gravity typically between 2.2 and 2.5, glass will feel light in the hand and is easily scratched by materials that a true sapphire would resist.