Sapphire is a variety of the mineral corundum, which is crystalline aluminum oxide (\(\text{Al}_2\text{O}_3\)). The sapphire gemstone is famous for its vibrant array of hues, most notably the deep blue color. These colors are caused by trace amounts of other elements, known as chromophores, that are incorporated into the crystal structure during its formation. Geologists rely on a suite of physical characteristics, like hardness and specific gravity, to accurately identify minerals, with the streak test being one of the more common methods for preliminary identification.
Defining the Mineral Streak Test
The streak test determines the color of a mineral when it is reduced to a fine powder. This powdered color, or streak, is often a more reliable property for identification than the mineral’s apparent body color, which can vary widely due to trace impurities or surface weathering. Many minerals, such as quartz or calcite, may occur in a variety of colors but will consistently produce the same color streak. The methodology for this test is simple, involving rubbing the mineral specimen firmly across the abrasive surface of an unglazed porcelain plate, commonly called a streak plate. The resulting fine line of powder left on the plate is the mineral’s true streak color. This test is particularly useful for opaque and colored minerals.
The Specific Streak Color of Sapphire
The streak color of sapphire is white or colorless. This result applies to all color varieties of corundum, including the intensely red ruby and the various “fancy” sapphires that occur in yellow, pink, orange, and green. It may seem counter-intuitive that a gemstone exhibiting such rich, saturated colors would yield a colorless powder. The vibrant blue of a sapphire, for instance, is a macroscopic effect of light interaction with iron and titanium impurities within the crystal structure. However, the streak test reveals the color of the mineral’s fundamental composition when powdered, which in the case of corundum, is essentially aluminum oxide.
Why Corundum Produces a Colorless Streak
The primary reason corundum produces a colorless streak relates directly to its extreme hardness. Corundum ranks as a 9 on the Mohs scale of hardness, making it the second-hardest naturally occurring mineral after diamond. The unglazed porcelain streak plate typically used for this test has a Mohs hardness of approximately 6.5 to 7. When a mineral with a hardness of 9 is rubbed against the much softer porcelain, the sapphire is not effectively ground into its own powder.
Instead, the corundum specimen largely scratches and powders the surface of the streak plate itself. Therefore, the white line observed is often composed of pulverized porcelain material, not sapphire powder, or the sapphire powder produced is so negligible that its color is diluted to white.
The secondary reason lies in the nature of corundum’s coloration. Corundum is classified as an allochromatic mineral, meaning its color is caused by trace elements, or chromophores, which are impurities rather than part of its main chemical formula (\(\text{Al}_2\text{O}_3\)). These chromophores are present in very low concentrations. When the mineral is powdered, the effect of these minute color-causing agents is insufficient to visibly tint the resulting fine, colorless aluminum oxide particles, thus yielding a white or colorless streak. This contrasts with idiochromatic minerals like hematite, which owes its color to a main chemical component and yields a distinct reddish-brown streak.
Distinguishing Sapphire from Other Minerals
The colorless streak of sapphire is a useful diagnostic tool, particularly when it is used to rule out certain softer impostors. Many common minerals that mimic the appearance of sapphire, such as colored glass or certain types of colored quartz, are significantly softer, with hardness values below 7, and may produce a colored streak or be easily powdered. The inability of a sapphire specimen to leave a distinct, colored powder on the streak plate immediately suggests a high hardness. This observation is a quick way to distinguish it from a mineral like lazurite, which appears blue but has a much lower hardness of 5 to 5.5 and a pale blue streak.
Ultimately, mineral identification relies on a combination of tests, as the colorless streak of corundum is shared by many other hard minerals. Geologists also examine properties like specific gravity, crystal habit, and cleavage. The streak test serves as an initial checkpoint, confirming that the specimen possesses the expected properties of a very hard, allochromatically colored mineral like sapphire.