Amethyst is the purple variety of quartz, a mineral whose chemical formula is silicon dioxide (\(\text{SiO}_2\)). Its distinctive color ranges from a pale lilac to a deep violet, making it a highly desirable gemstone. Because of its popularity, the market frequently contains imitations, ranging from colored glass to synthetic crystals. Identifying a genuine specimen requires a systematic examination of its visual traits and physical properties.
The Signature Visual Traits
The defining characteristic of amethyst is its purple coloration, caused by trace amounts of ferric iron (\(\text{Fe}^{3+}\)) impurities within the quartz crystal structure. This iron, when exposed to natural irradiation, forms color centers that absorb light and produce the violet hue. The stone’s color intensity varies significantly, from the light “Rose de France” shade to the highly saturated “Deep Siberian” purple.
A significant trait to observe is color zoning, the uneven distribution of purple within a single crystal. Natural amethyst often shows bands, patches, or streaks of varying color intensity, sometimes with angular or straight boundaries. This zoning reflects changes in the mineral’s chemical environment during its formation. Imitations, such as dyed glass, typically lack this natural variation, presenting a uniform shade of purple.
Amethyst crystals commonly exhibit a six-sided prism shape, often terminating in a six-sided pyramid. In nature, it is frequently found lining volcanic rock cavities, forming geodes or druzy clusters. The mineral is transparent to translucent, allowing light to pass through with varying degrees of clarity.
Confirming Identity Through Physical Tests
The most reliable test for amethyst involves assessing its hardness, a property measured using the Mohs scale. Amethyst, a variety of quartz, registers a hardness of 7 out of 10. This high number indicates a strong resistance to scratching compared to many other common materials.
Practically, a genuine amethyst specimen can scratch materials with a lower Mohs rating, such as glass (Mohs 5.5) or a steel knife blade. Conversely, if the specimen can be easily scratched by a copper penny or a common steel tool, it is likely not true amethyst. This test is an effective, non-destructive way to verify the mineral’s identity, especially against softer imitations.
Beyond hardness, genuine amethyst displays a vitreous, or glassy, luster. When broken, it exhibits a conchoidal fracture, meaning the surface is smooth and curved, resembling the inside of a seashell. Amethyst does not possess cleavage, meaning it will not break along flat, predefined planes.
Distinguishing Amethyst from Look-Alikes and Fakes
Several materials can be mistaken for amethyst, but their differing physical properties allow for clear separation. Purple fluorite is a common look-alike, but its Mohs hardness is only 4, meaning amethyst can easily scratch it. Fluorite belongs to the cubic crystal system, whereas amethyst is trigonal.
Colored glass is a frequent imitation, but close inspection often reveals signs of its artificial origin. Genuine amethyst, even with internal imperfections, will not contain the small, spherical air bubbles characteristic of glass fakes. Glass also lacks the natural color zoning typical of amethyst specimens.
The color of amethyst is sensitive to heat, which provides another way to distinguish it from heat-stable synthetics. When exposed to high temperatures, the purple color can fade or change completely, sometimes turning yellow-orange to create citrine. This thermal instability is a characteristic of natural amethyst not shared by imitation materials.