There are indeed some rare and fascinating types of rocks that can change color. This ability is due to the unique chemical composition and crystal structure of these rocks. When exposed to certain environmental conditions, the minerals within the rocks undergo chemical reactions that cause their colors to change. The most well-known examples of color-changing rocks include opal, labradorite, and lepidolite. In this article, we will explore what makes these rocks able to shift their hues and the circumstances that trigger their remarkable transformations. Understanding the science behind these shape-shifting stones reveals the hidden beauty and complexity within the geological world.
What is opal and what makes it change color?
Opal is a type of silica mineraloid that contains 3% to 30% water within its structure. This water is trapped inside tiny spheres and cavities in the opal. The varying size and arrangement of these water-filled pockets determine the color patterns exhibited by opal stones. When white light hits opal, the water diffracts the light and breaks it into different spectral colors. Smaller water pockets scatter blue light waves, while larger pockets scatter red wavelengths. This diffraction of light by the internal water and silica structure causes opals to flash a rainbow of colors as the viewing angle changes.
Some varieties of opal also exhibit play-of-color, which is the ability to shift between shades of green, blue, red, and orange. This effect is caused by the ordered arrangement of silica spheres that diffract light. If the opal dries out and loses water, the play-of-color will disappear. Opal can reversibly gain water and change color again. Heat, dryness, and sun exposure are environmental factors that impact the water content and cause color change in opal gems.
What is labradorite and what makes it change color?
Labradorite is a type of feldspar mineral that belongs to the plagioclase feldspar family. It displays an optical phenomenon called labradorescence, which refers to the colorful iridescent flashes apparent on the surface of labradorite stones. These flashes can shift between blue, green, yellow, orange, red, and purple hues.
Labradorescence is caused by lamellar twinning in the crystalline structure of labradorite. This describes a layered arrangement of intergrown mineral grains with different refractive indexes. When light hits these lamellae, it is refracted and separated into the various spectral colors. The color observed depends on the thickness of the lamellae and the viewing angle.
At certain angles, the play of color is extinguished. Heating labradorite to 600-1000°C alters the twinning structure and destroys labradorescence. The color change ability can also be impacted by weathering when the surface crystals are altered or damaged. But interior crystals retain the lamellar structure and schiller effect.
What is lepidolite and what makes it change color?
Lepidolite is a lilac-gray or pink member of the mica mineral group. It contains a significant amount of lithium as well as other metals like manganese, iron, magnesium, and aluminum. The lithium content causes lepidolite to exhibit interesting color properties.
The manganese in lepidolite can occur in two oxidation states – Mn(II) and Mn(IV). Mn(II) ions give a violet-pink color, while Mn(IV) shows little color. When lepidolite is exposed to oxygen, humidity, or heat, the Mn(II) ions give up electrons and are oxidized to colorless Mn(IV). This causes the pink lithium mica to fade to a pale gray.
Conversely, reducing Mn(IV) back to Mn(II) will restore the pink coloration. This can be achieved by heating lepidolite in a vacuum or inert atmosphere. The color change is reversible and repeatable with oxidation/reduction reactions. Weathering releases the unstable Mn(II) and leads to white discoloration on lepidolite surfaces. But the core often retains lavender tones.
Other color-changing rocks
While opal, labradorite, and lepidolite exhibit the most striking color changes, there are a few other minerals and rocks that can alter their hues under certain conditions:
| Mineral/Rock | Color Change | Reason for Change |
|---|---|---|
| Fluorite | Purple, green, blue, yellow to colorless | Exposure to shortwave ultraviolet light causes fluorescence |
| Ammolite | Orange, red, green to gray | Loss of water causes loss of iridescence |
| Aragonite | Blue, green, red to white | Oxidation of mineral causes discoloration |
| Carnelian | Red/orange to brown or yellow | Bleaching by sun exposure |
The color changes primarily occur due to oxidation, reduction, dehydration, and irradiation effects on the chemical structure of the minerals. Understanding these mechanisms allows geologists to gain insights into the formation history and environmental conditions experienced by different gemstones and rocks.
Conclusion
While most rocks appear to have a stable and permanent color, there are some intriguing examples like opal, labradorite, and lepidolite that can transform before your eyes. The color shifts displayed by these rocks depend on their unique chemical makeup as well as exposure to stimuli like light, heat, and humidity. The interplay between chemistry and environment gives rise to the remarkable, shape-shifting nature of these minerals. So in your rock collecting and gem hunting endeavors, keep an eye out for these chameleonic stones that radiate mystifying iridescence and color variety. Their changeable beauty is a testament to the enduring wonder of the geologic realm.
