Amethyst, a purple variety of quartz, is known for its beautiful violet hue. Prolonged exposure to sunlight can cause the color to lighten or disappear entirely. This change occurs because of the interaction between the sun’s energy and the crystal’s structure.
The Science Behind Color Loss
The distinct purple color of amethyst originates from the presence of trace amounts of iron impurities within the silicon dioxide crystal lattice. During the natural formation process, these iron atoms are altered by natural gamma radiation, creating unstable “color centers.” This change in the iron’s oxidation state allows the crystal to absorb specific wavelengths of light, producing the characteristic purple coloration the eye perceives.
However, this colored state is inherently unstable when exposed to surface conditions. Ultraviolet (UV) radiation present in sunlight provides the energy needed to disrupt these color centers. The UV energy causes a photochemical reaction, essentially reversing the process that created the color in the first place. The iron reverts back to its original state, leading to a loss of the vivid purple color. This process, sometimes called solarization, can eventually leave the gemstone colorless or with a grayish tint. The exact rate of fading depends on the intensity and duration of the sun exposure, as well as the concentration of iron impurities in the specific stone.
Fading Versus Heat Treatment
It is helpful to distinguish between the slow, unintended fading caused by UV light and the rapid, intentional alteration achieved through high heat. Sunlight causes a gradual, destructive bleaching of the purple color over months or years. This slow process is considered damage, and the original color cannot be restored once the fading has occurred.
In contrast, the commercial jewelry industry frequently uses controlled heat treatment to purposely change the color of amethyst. When heated to temperatures typically ranging from 400°C to 500°C, the iron impurities within the quartz lattice are structurally altered. This thermal alteration often turns lower-quality purple amethyst into yellow or orange citrine.
A small number of amethyst deposits contain material that turns a yellowish-green to green color at around 500°C, which is marketed as prasiolite. The intentional heat change is a permanent transformation, yielding a new gemstone variety.
Practical Care and Storage
Protecting amethyst specimens and jewelry requires minimizing their exposure to both prolonged sunlight and intense heat. The most effective preventative measure is to store or display the gemstone in a cool, dark environment. This means avoiding placement on sunny window sills, in vehicles, or near heat sources like fireplaces or heating vents.
Cleaning Amethyst
When choosing cleaning methods, use only warm, soapy water and a soft brush. Methods that introduce high heat, such as using a steam cleaner, should be strictly avoided as they can unintentionally cause the color to change or fade.
Display and Wear
Jewelry worn outdoors should not be left in direct sunlight for extended periods, such as when sunbathing. For display purposes, using cases with UV-filtering glass can help mitigate the damaging effects of indoor and outdoor light sources.