The ruby is a captivating gemstone, known for its deep red color, making it one of the most highly valued minerals throughout history. It is a variety of the mineral corundum, defined by its core chemical composition of aluminum oxide (Al2O3). The striking red hue is caused by trace amounts of chromium substituting for aluminum within the crystal structure. Identifying this mineral relies on examining its physical and optical characteristics, including its mineral streak.
Defining the Mineral Streak Test
The mineral streak is a diagnostic property used by geologists, representing the color of a mineral when it is ground into a fine powder. This test is performed by dragging a specimen across an unglazed porcelain plate, known as a streak plate. The resulting mark is a line of pulverized mineral fragments left behind on the ceramic surface.
The scientific principle is that the powdered form provides a more consistent color than the bulk mineral itself. A mineral’s outward color, called its body color, can be highly variable due to surface impurities or trace elements. When crushed, the crystal structure is destroyed, making the color less dependent on how light interacts with the crystal lattice. This process reveals the inherent color of the mineral’s primary composition, making the streak a reliable identification tool.
The Specific Streak of Ruby
The theoretical streak of a ruby, a variety of corundum, is white or colorless. This result directly reflects its base chemical composition, aluminum oxide. In its pure form, corundum is transparent and lacks color, which dominates when the crystal is pulverized.
Chromium, responsible for the ruby’s vibrant red body color, is present only as a trace element, not a major structural component. When the ruby is reduced to a powder, the concentration of this color-causing impurity is not high enough to impart a noticeable red color. Therefore, the resultant streak is simply the color of the base mineral.
Limitations of the Streak Test for Hard Minerals
While the streak test provides a specific color result for ruby, this observation is theoretical and not a practical identification method. The test’s effectiveness is limited by the relationship between the mineral’s hardness and that of the streak plate. The standard unglazed porcelain streak plate has a Mohs hardness of approximately 6.5 to 7.
Ruby ranks 9 on the Mohs scale of mineral hardness, second only to diamond. Since ruby is significantly harder than the plate, attempting the streak test results in the ruby scratching the porcelain surface. The white line left behind would be powdered ceramic material, not pulverized ruby. Furthermore, applying this test to a valuable, faceted ruby would be a destructive act, confirming its impracticality.
Primary Methods for Ruby Identification
Due to the limitations of the streak test, gemologists rely on non-destructive and highly specific methods to identify and authenticate rubies. The exceptional hardness of 9 on the Mohs scale is a useful initial indicator, as few other red gemstones share this property. For instance, common ruby simulants like red spinel and garnet are softer, ranking between 7 and 8.5.
A more precise method involves measuring the refractive index (RI), which determines how light bends as it passes through the gem. Ruby is a doubly refractive mineral with a characteristic RI range of approximately 1.76 to 1.77, measured using a refractometer. This test quickly distinguishes ruby from look-alikes that possess different optical properties.
Other Identification Techniques
Another important measurement is specific gravity, the ratio of the gemstone’s density to the density of water. Rubies consistently exhibit a specific gravity value between 3.97 and 4.05 because corundum is a dense mineral. Microscopic examination is also performed to identify internal features such as characteristic inclusions. The presence of fine, needle-like structures of rutile, known as “silk,” confirms a stone’s identity and natural origin, separating it from synthetic materials or glass imitations.