Crystals & Specimens – Gem and Geode

It was in my early collecting days that I noticed my amethyst beginning to change color and fade. I couldn't figure out why. Now I am the wife of a geologist and will explain when and why amethyst changes color and how to prevent it! There are three reasons that amethyst changes color or fades: heat treatments, sunlight (UV), and water containing certain minerals. Stop your amethyst from changing color or fading by avoiding heat treatments, limit time spent in sun or UV light, and never clean amethyst with hard tap water. "But I love my amethyst, the color is just perfect the way it is!". Read with me a bit further to explore why this happens and learn how to protect your amethyst from these culprits. Why do Heat Treatments Change the Color of Amethyst? Some people may choose to heat their amethyst with the intention of altering its color hue. Amethyst can be exposed to high temperatures for several hours, known as a heat treatment. Amethyst will turn from purple to yellow, orange, brown, white, and sometimes even a pale green. Heat treatments are done intentionally to change the color of the amethyst for aesthetic purposes. So, why do high temperatures cause amethyst to change color? In 2020, Renping Cheng & Ying Guo tested the idea that these color changes weren’t actually because of a true chemical reaction. They discovered that the iron and the oxygen atoms within the crystal swap electrons and this is what changes the amethyst’s color. The higher the temperatures and longer the exposure to heat, the more electrons that are swapped, resulting in more dramatic color shifts. Table and image comparing heat treatment temperature to resulting color. Temperature (℃)Perceived Color360℃Amethyst Purple400℃Pale Purple440℃Cool Grey480℃Warm Grey520℃Off White560℃Khaki600℃Light Tan800-900℃Synthetic Citrine Heat treated amethyst is not universally accepted within the jewelry community. As it is a permanent treatment, it forever alters the stone from its original color. While some say that it improves the color, others disagree and prefer to preserve natural beauty. I personally love the natural colors and complexities of amethyst and prefer my gemstones and crystals to be untreated. An additional implication that comes with heat treating amethyst is the creation of “burnt amethyst”, which has a similar color to citrine. Because of the similarity in color, burnt amethyst can easily be confused for citrine. If you are concerned about purchasing a heat treated gemstone or crystal, ask your vendor about disclosed heat treatments to be sure the color is natural. Why Does Sunlight (UV) Change the Color of Amethyst? Sunlight and UV work in a similar way to heat when changing the color of amethyst. The natural purple color of amethyst is defined as a wavelength (545 nm), which is the predominant one of many wavelengths in amethyst’s color band. Virtually all gems and crystals have what some refer to as characteristics or fingerprints. One such special feature is called a color center. Color centers are atomic defects within the stone's crystal structure that filter light a little differently. When areas form, they can change the wavelength of light to make a different color. While typically beautiful, they are a double edged sword as any sunlight passing through contains UV rays. Over time and with repeated exposure, the same process of electron swapping between the iron and oxygen elements occur. This can lead to unexpected changes of color in your stone. "NEVER use UV cleaning devices on amethyst! They can fade your amethyst over time."Joshua Solberg, Gemologist & Geologist Photo by Rémi Bornet from Le Comptoir Géologique. There are several steps you can take to protect your amethyst from the sunlight and UV exposure. Firstly, never store your rocks or crystal collection next to a window. If you ever find your amethyst losing color, check where you are keeping it. It may be getting more light during the day than you realize. While having the sunlight shine through your gems and crystals looks phenomenal, amethyst is not alone when it comes to reacting with light and UV. This includes LEDs, as researchers from Le Comptoir Geologique found. Because of this, if you have lights in your display cases, consider the rocks and crystals you store in them carefully. It is wise to ensure that those specific crystals will not fade over time with exposure to light. Alternatively, you can limit the amount of time the display cases are lit to special occasions. Short, limited exposure will minimize any changes in your amethyst for the next century. New devices that have been gaining some popularity are UV sanitizers. While convenient, the high exposure to UV makes them not ideal for sanitizing your amethyst. Why Can Minerals in Tap Water Change the Color of Amethyst? Do you have patches of water that dry white in your sinks or shower? This is especially visible on metallic surfaces. These deposits are the minerals left behind when the water evaporates. When washing your favorite crystal a quick rinse can be all it needs to restore its luster, but remember what happens to the sink? The same could happen with your stone if you have “hard water” like Geode and myself. If water enters a crack, it could leave these visiting minerals behind. If enough minerals build up inside the crack, parts of your stone could become white and dull. What can you do instead? Using distilled water and gently drying your wet crystal with a cloth will stop the water from leaving deposits. However, there are other substances within water that could be left inside some cracks on the surface of a stone. Those of you who take baths with your crystals, your bath water can contain salts, chemical compounds, and oils. The ocean and many lakes also contain salt. While typically not harmful, I always advise care to keep your gems and crystals in their most brilliant state. The Good News! Now for the good news! Most cracks that form inside amethyst are extremely thin and won’t allow even air to enter. It takes immense strain or force to cause a crack within amethyst for a specific reason. Amethyst is a type of quartz, which is one of the hardest and most common natural materials. They can weather through the toughest environments for eras. Even when ground down over eons of time by rubbing back and forth in water against other crystals of its kind, quartz never really goes away. Stay Brilliant! - Gem References Cheng, R., Guo, Y. Study on the effect of heat treatment on amethyst color and the cause of coloration. Sci Rep 10, 14927 (2020). https://doi.org/10.1038/s41598-020-71786-1 https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Electronic_Spectroscopy/Metal_to_Ligand_and_Ligand_to_Metal_Charge_Transfer_Bands https://www.rp-photonics.com/color_centers.html