Amethyst is a widely recognized purple gemstone, valued for its color and clarity in jewelry and ornamental objects. While its vivid hue is its most distinguishing feature, geologists rely on a suite of physical tests to definitively confirm a specimen’s identity. These diagnostic procedures ensure that the purple crystal is truly amethyst and not another mineral that shares a similar external appearance.
The Role of Streak in Mineral Identification
The mineral streak is the color of the finely powdered mineral left behind when it is scraped across an abrasive surface, typically unglazed porcelain (a streak plate). Unlike the mineral’s overall body color, which can vary due to impurities or weathering, the powder’s color remains consistent. This consistency makes the streak test a reliable diagnostic tool for mineral identification. The test is most informative for opaque and highly colored materials that are softer than the streak plate itself.
Amethyst’s Specific Physical Characteristics
The streak of amethyst is white or colorless, a characteristic shared by all varieties of quartz. Amethyst registers a 7 on the Mohs scale, making it harder than the porcelain streak plate (typically 6.5). A mineral harder than the plate will scratch the surface instead of leaving a true powdered streak. Therefore, the white streak is not the most definitive characteristic for quartz, requiring other physical properties for positive identification. Geologists use the mineral’s high hardness of 7 as a stronger indicator.
Amethyst also demonstrates a lack of true cleavage, meaning it does not break along distinct, flat crystallographic planes. When broken, the mineral instead exhibits a conchoidal fracture, resulting in smooth, curved surfaces resembling the interior of a seashell. These properties, combined with a vitreous or glassy luster, are used to confirm the presence of amethyst.
Composition and Formation of Amethyst
Amethyst is classified as a variety of quartz, sharing the basic chemical formula of silicon dioxide (\(\text{SiO}_2\)). The purple coloration results from trace amounts of iron ions incorporated into the crystal lattice during formation. Amethyst crystals form in hydrothermal veins or within hollow cavities, known as geodes, often found in volcanic rocks like basalt.
The full purple hue develops after the iron-containing quartz is exposed to natural gamma radiation over long periods. This radiation alters the electronic structure of the iron ions, creating color centers. These centers selectively absorb light wavelengths from the yellow and green parts of the spectrum. The remaining transmitted light is a combination of red and blue, resulting in the characteristic violet-purple color.