Crystal Color Zoning

Crystal color zoning is a fascinating phenomenon that occurs when different colors appear in bands or patches within a single crystal. It essentially refers to the presence of distinct color bands or patterns within a single crystal. These bands can vary in width, intensity, and sharpness, creating visual effects that range from subtle to strikingly dramatic. This variation in color is a result of changes in the crystal's chemical composition during its formation.

Crystal color zoning, also known as color banding.

Crystal zoning is a texture developed in solid-solution minerals and characterized optically by changes in the color or extinction angle of the mineral from the core to the rim.

Crystal zoning A texture developed in solid-solution minerals and characterized optically by changes in the colour or extinction angle of the mineral from the core to the rim. This optical zoning is a reflection of chemical zoning in the mineral. 

For example, a plagioclase can be zoned from a Ca-rich core to an Na-rich rim. Zoning results from the mineral's inability to maintain chemical equilibrium with a magma during rapid cooling; the zonation represents a frozen picture of the continuous reaction series for that mineral.

Crystal Zoning. Beautiful Zoning Purple Fluorite Crystals from Minerva Mine, Hardin Co., Illinois, USA Photo credit: Anton Watzl

Types of Color Zoning

Concentric Zoning: The most common type, where bands of different colors appear like rings around the core of the crystal. This reflects changes in the composition of the surrounding fluid or magma as the crystal grew. For example, in tourmaline, green zones might indicate periods of higher iron availability, while pink zones signal lower iron and higher manganese.

Sector Zoning: Color variations follow specific crystallographic directions, resembling sectors or wedges within the crystal. This zoning often arises from uneven distribution of trace elements during crystal growth.

Oscillatory Zoning: Tiny bands of alternating colors appear throughout the crystal, resembling growth rings in trees. This type of zoning suggests rapid fluctuations in the surrounding environment, with alternating phases of favorable and unfavorable conditions for specific color-causing elements.

The salt deposits of a salt pan are zoned like bathtub rings, with less-soluble sulfates and carbonates at the outer margin and highly soluble sodium chloride (table salt) at the centre. The crystallization of these salts can be compared with the evaporation of brine in a dish.

Causes of Color Zoning

Changes in temperature or pressure: As the temperature or pressure surrounding the growing crystal fluctuates, the solubility of different elements can change, leading to variations in their incorporation into the crystal structure.

Variations in chemical composition of the surrounding fluid or magma: If the composition of the surrounding fluid changes due to interactions with other minerals or fluctuations in the magma chamber, the available elements for incorporation into the crystal can also vary, leading to color zoning.

Presence of trace elements: The availability and incorporation of trace elements such as iron, chromium, manganese, and vanadium play a crucial role in determining the color of many minerals. Changes in the availability of these elements during crystal growth can lead to distinct color zones.

Various examples of growth zoning features typically found in hydrothermal fluorites

No apparent zoning (A) suggests crystallization under constant hydrothermal conditions, whereas concentric zoning (B) indicates discontinuous changes in the fluid chemistry over time; (C) is a typical representation of short-lived oscillating changes in the fluid chemistry. (D,E) Typical sectoral zoning in fluorite crystals with complex crystal habits ((D): cuboctahedral, (E): cubic + rhombic dodecahedral faces) that is only visible in specific plane sections. (F) Combination of sectoral and oscillatory zoning in a cubic fluorite crystal. (G) 'Fir-tree' zoning occurs when growth conditions (T/Eh) alternate over time and thereby favour different crystal morphologies (cuboctahedron ↔ cube), which proves pulsed crystallization (e.g., [106]). (H) 'Petal-type' zoning is the result of a continuous change from octahedral growth to cubic growth; the red line marks the beginning of shape transition (modified from [104]).

Significance of Color Zoning

Provides insights into crystal growth history: By studying the type and pattern of color zoning, geologists can gain valuable information about the conditions under which the crystal formed, such as temperature variations, the composition of the surrounding fluid or magma, and the growth rate of the crystal.

Adds to the beauty and value of some gemstones: In certain crystals like amethyst, watermelon tourmaline, and certain sapphires, color zoning can add a unique and captivating element to their beauty, enhancing their value and desirability among collectors and gem enthusiasts.

Can be used for gemstone identification: In some cases, the type and pattern of color zoning can be diagnostic of a specific mineral or gem variety, aiding in identification.

Crystal Color Zoning Examples

Watermelon tourmaline: This gemstone exhibits classic concentric zoning with green and pink bands.

Amethyst: Some amethyst crystals have a colorless core surrounded by a purple zone, representing a change in iron content during growth.

Fluorite: This mineral can display a stunning array of color zones, with bands of purple, green, yellow, blue, and even black creating breathtaking patterns. Different types of fluorite have specific zoning patterns, like "Blue John" showing concentric zoning with purple and blue bands.

Agate: Known for its intricate banding patterns, agate also exhibits color zoning within these bands. Colors like red, orange, blue, and green can intermingle, creating stunning landscapes and scenes within the gemstone.

Conclusion

The presence and type of color zoning can have a significant impact on the value of a gemstone. In some cases, zoning can detract from the gem's value, particularly if it creates uneven color distribution or dull patches. However, in other cases, zoning can enhance the gem's beauty and value, especially when it creates interesting patterns or contrasts. For example, some amethysts with well-defined color zoning are highly sought-after by collectors.